Quick question about adding RAM

Hello, my computer is a Acer Aspire 5742G, everything is as it was bought but recently due to overheating issues I dissassemblied it and cleaned the fan. Since then (or maybe a while after, I'm not quite sure), I started noticing it was getting very slow so I paid attention to my RAM and noticed only 2GB was being used (Computer Properties and Task Host say so) from the 4GB built in when purchased. I tried switching the RAMs' slots between themselves but only 2GB was being used still. I'm assuming I damaged one of the RAMs when I dissassemblied the computer so I'm going to replace it.

My questions are:

1-Is there any other reason why it's only using 2GB or am I right in assuming I damaged one of the modules?

2-Could I buy a 4GB module and have it pair with my working 2GB one? Then I'd have 4GB+2GB=6GB RAM. I haven't made up my mind whether I should or not buy a pair of 4GB modules so this could be a nice alternative.

Thank you in advance :)