Easy replacement job but do you know if this machine has had the BDI mod fitted to it? Issue might not be your batteries, these Wave 50's needed a BDI mod kit fitting as they had issues with BDI readings when batteries were actually ok.

It's an easy mod, just a few relays and a couple of wires from line contactor into can bus system.

I'd be checking that out first before you throw 4 batteries at it, unless you've already load tested them and they are buggered.