PaulJV

@TaliaB  I have charged all batteries individually to 100% and it seems like the problem is solved. I have also installed a 48v equiliser. Thank you again for your assistance.

Have you tested the 4 batteries and isolated the one not delivering power? 
Looks like you have 3 flat 12v batteries? giving almost 36v
I would expect if they were all fully charged to get a voltage around 13.3v from each battery if fully charged? maybe a bit less, check on the side of the battery or manual for the specs?
What are the maximum charge and discharge currents for the batteries? Check the inverter is set to not discharge / charge to fast? Being 200ah I would expect it to be 20-30 amps max. for charging, however it may tolerate a slightly higher discharge rate for a few seconds? Again the manual should have those specs?
Many Lithium battery packs with a BMS are designed for only a certain nominal voltage and cannot be connected in series. in other words do not connect 2 x 24v battery packs together to give you 48v. Not sure if this applies in your case, again I would check with the Manual or supplier?
 
I also noticed these appear to be second life batteries? 

 
Found these specs online, not sure if it is the same product you have?
https://www.mecerpc.co.za/Mecer-SOL-B-L-M200-p-239794.php
SOL-B-L-M200
Mecer Second Life LIFEPO4 Lithium Battery 12.8V 200Ah
Mecer second life LIFEPO4 Lithium Battery 12.8V 200Ah, 1500 charging cycles, Can use up to 4 batteries in series with the built in BMS (Battery Management System) **6 month warranty or rated cycles, whichever comes first
Benefits of Lithium solar storage / power backup batteries.
Lithium Batteries have a longer life span than Lead Acid Battery Lithium Batterieshave deeper discharge cycles than Lead Acid Battery Lithium Batteries stay above 12.5V at 90% discharge Lithium batteries are lighter for the same capacity (about 33% of the weight of Lead Acid)
Specifications:
Normal capacity: 200Ah Voltage: 12.8V Energy: 2304Wh Charge Voltage: 14.6V Discharge cut-off voltage: 10V Charge Method: CC/CV Standard Charge current: 20A Max. Charge Current: 100A Max. Continuous Discharge Current: 100A Cycle life:1 500 times Internal Impedance: <50mOhm Dimensions: L521 x W237.9 x G217.3mm Weight: 23Kg Working temp: 0 - 45deg C Storage temp: -20degC to 60degC BMS (Battery management system) included to extend the life-time of the battery (up to 4 batteries in series)