sakmaz

i wouldn't go to through that trouble either ,just use them till they dont satisfy you anymore then go for a LFP battery(s).

It is normal for the voltage to drop a little when the charging source is disconnected, the battery will usually settle below the float voltage.
The most important with lead acid batteries is the cut off voltage, make sure this is at lesat 48V or above. A low cutoff voltage is what kills lead acid the most and the default on most inverters is too low. If you are only using 10% in most cases then you would probably not go below 48V, but with multiple load sheds you might and over time that will kill the battery, so keep an eye on how low you are going.
I would agree with @Nexuss, I would keep using them until they die, they still seem to be doing fine and serving you well, no need to replace them until then and likely lithium battery prices will be cheaper in future.
I have a set of 125Ah flooded lead acids that have been going since 2016 and a set of 200Ah gels that are 4 years old now and still doing fine, these batteries can last many years with low loads like you have.

When the batteries are dying you will see it during a 4.5h LS period. 
It is a bit of a time consuming job without the right equipment. 
It will be normal for the Gels to drop to about 50-51V at a 500W load during LS. 
When they are fully charged you can isolate them. Disconnect them. Take the 4 to a Battery Centre and ask them to do a test. This way they can give their input on state of health.