"when i turn on the charger it seems to be charging
but only to 9.20v however when the motor is running it shows 10.80 v."
Voltage levels that low indicate either a dead battery or extensive corrosion
on the wiring
I'd suggest "divide and conquer". Buy a $20 multimeter at WalMart or Target, that's good enough to start with. Using the 20VDC scale, test the output of the battery charger, all by itself with no battery connected. Most will read 15-17VDC with nothing else connected. Get the manual for that charger, because some of the newer ones are too smart, and they won't charge a deeply discharged battery at all.(Because there is some danger
Once you know the charger is good, test each battery, See if any read above 12VDC. Try to charge one, and just one, at a time, and charge it directly with the other batteries not connected. This will tell you if one or more batteries are shot or if there is a wiring
Use the hydrometer, and buy the big one (12" not 4") when you do. It will try to spit or drip battery acid, so wear old clothes and expect acid burns in them no matter how careful you are. I keep mine in a plastic case because no matter what you do with it, it either smashes or scatters acid drops unless it is cased up somehow.
All the terminals and cables
should be clean--clean enough to eat off them. If not, clean them. If any of the cables
are not tinned wire, and they show green crud at the ends? They need to be replaced.
That should give you a start on it. Four batteries at once is a lot to troubleshoot. Divide and conquer, one is enough to start on.