BigC

I wouldn't do that as a first choice. The inverter can in theory connect up to 10,400W of panels. Now I understand that it's a mixed bag of panels, but I'd start by maybe listing what you've got by make and model of the panels, and then seeing if one can come up with a configuration that could allow you to add that 4kW of panels without resorting to another MPPT. If it's a forlorn hope, if it doesn't work, then so be it, or if you've already gone through the thought process and concluded that you need the MPPT solution, that's something else, but from what you've written, I'd at least ask if this is an option.

A Solis grid tie inverter on the Aux port with frequency control is better than a standalone MPPT.

Copied and pasted
Methods for Adding External MPPT to Deye
There are two primary ways to add more solar capacity to an existing Deye installation:
Direct-to-Battery (External MPPT/Charge Controller): You install a separate, third-party MPPT charge controller (e.g., Victron SmartSolar) and connect its output directly to your battery bank.
Pros: Independent of Deye MPPT limitations; easiest for adding panels with different voltages or orientations.
Cons: Requires its own monitoring/app; not directly visible in the Deye Solarman app.
AC Coupling (Additional Grid-Tie Inverter): You install an AC-coupled microinverter or string inverter and connect its output to the GEN or GRID port of the Deye inverter.
Pros: Fully integrated into Deye monitoring; efficient for large scale expansion.
Cons: More complex setup, requires proper grid-tie settings

No, you need to run identical inverters in parallel.
Buying a hybrid inverter and not using with a battery is a waste of money. If you couple both inverters to the batteries, you will have double the losses/own consumption.

Yes, this is the AC coupling method as described above, and I guess you can read more about in the manual of the Deye under the Grid/Aux port operating modes. Basically the Deye inverter acts as a controller and creates a mini-grid. The Solis inverter will respond to the grid signal from the Deye, generate power and the Deye inverter will then charge the battery connected as normal.

Thanks @Sc00bs
Looks like the answer to a maidens prayer. I will report back once I've tried this . . . Ciao

So-called hybrid, and/or off/on-grid inverters have evolved into a plethora of types that is quite bewildering .. and there are definite followers of the various brands, I guess driven by their experience and familiarity with them. Even the word hybrid has come to mean different things to different people. In an attempt to understand their inner workings I produced a few diagrams to describe these inner workings, based on studying the specifications and to a greater part, the experience of the many contributors to the forum.
As my understanding grew, the diagrams reduced to only two .. the Voltronic type, which is a pure off-grid inverter, and the Deye type, which is both off-grid and on-grid. The two types being differentiated by their firmware functions and the sophistication of the hardware blocks. 
Despite no mention in the documentation, the Deye seems to have a bypass function same-size the Growatt (had some experience with both of these inverters). I witnessed this by measuring the Deye inverter output before and after the grid input was killed. No doubt the inverter outputs the higher 245V here in the Overstrand rather than the programmed 230V, when On-grid. 
AFAIK, this bypass is permanent. In the event that the grid fails, the grid is disconnected and the AC/DC output inverter block (which is permanently connected, powers the output. There is a Victron diagram which shows this very well. 
I'm pretty sure they all feature an internal DC bus at a high voltage (as in the Growatt) that is required by the AC/DC inverter block to manufacture the 300Vpk voltage of the sinewave output. The bus is powered by:
1. the DC/DC MPPT converter(s)
2. the DC/DC bi-directional battery converter
3. the bi-directional AC/DC grid inverter. 
I guess the ability to blend power from the various blocks, is predominantly enabled by the firmware, not the hardware, as I imagined. This final stage is the bi-directional inverter .. the magic all happens within this block. This stage drives the AC load output, but when programmed appropriately, can drive the grid connection as well. The CT connected to the grid input is what enables the inverter to power devices on the grid (or non-essential load) side, without pushing back onto the grid itself.  The Aux port is merely a switched configuration of this inverter block.
Methinks that output inverter block, and the sophisticated firmware required to drive all the electronics is what you pay the big bucks for.
I write this in an attempt to promote a clearer understanding of the different inverters, and sincerely look forward to feedback, criticism, correction, or whatever, to this article. No doubt there are many vastly more experienced and knowledgeable folks out there who may have different opinions, or information to add.

Oh noooo .. there is an error in the first diagram. It should read "Voltronic type inverters" .. sorry 😬

Hi folks
The Sunsynk/Deye inverters are bi-directional hybrid inverters (which terminology is often misused) in that they can be configured to feed excess PV energy back to the grid side (or non-essential-load side), much like a grid-tie inverter, simultaneously with powering the load output and charging the battery. A small CT installed at the incoming grid/muni connection allows the inverter to back off and not back-feed into the grid. This is what I understand, but have no direct experience of, so be gentle with me if I have it wrong.
IMHO this is a game changing feature. It means that you can power your geyser with excess PV energy, without placing it on the essential-load side of the inverter. Hence it will never draw power from your battery, or load the output of the inverter, and will draw the balance of energy required to heat the water from the grid.
So Eldred .. If you have one of these inverters and PV panels then you have all you will need to heat you geyser economically .. no need for a separate solar-thermal/pumped system, PV geyser system, heat-pump or whatever. Effectively the battery is just for load-shedding. Rather buy more panels, as and when you need/can afford them.
What is seldom if ever mentioned on this forum is that various municipalities charge a monthly levy just for the privilege of having an electrical connection (which can be substantial, and can never be cancelled), and even a monthly levy for a grid-tie connection (in-feed tariff applicable to read the meter .. also substantial), which fees all skew the financial model for your system. For this reason, so many members of this forum recommend that you stay on the grid, and don't apply for the in-feed facility.
I don't have such a system, and sure wish I did. There is so much scope to play.