Possibly an apartment number, although I would expect it to be on the third floor.
What does 3a mean in this address?
Sometimes a large apartment is divided into two smaller apartments with separate doors.
If the original one was #3, the second one would be #3a.
If it is apartment 3A, then I would not use the slash: 3A 4th Floor.
I would probably separate them with a comma (or put them on separate lines).
Mr J Smith
Flat 3A, 4th Floor
125 Florence Road
To be honest, if it were a British address, there would be no need to mention which floor it's on. Flat/Apartment 3A is flat/apartment 3A, no matter which floor it's on. It seems unlikely that there would be a "Flat 3A" on several different floors of the same building.
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.