2000 electric club car needs more power

HI all! New to forum so be kind lol. I bought a 2000 48 volt Club Car and rehabbed it. It needed batteries so I put in 4 12 volt deep cycle batteries in series to get the 48 volts. Probably should have went the 6 8 volt batteries but money was getting tight. The cart runs fine on level ground but going up hills it seems low on power. I have cleaned all the electrical connections so I know that's not an issue. I probably could have put in bigger batteries but didn't. My question is this - Can I add another 12 volt battery in parallel to boost the amperage? I'm not sure if it would damage the motor or the 48 volt charger.

Thanks in advance for your replies.

Bob