It's probably using all of your available RAM if your system only has 4 GB. Windows takes up some RAM, so does the frame buffer. I imagine some data was being off loaded to virtual memory (the HDD) if you were using 3.8 GB.
If you only have a single stick of RAM right now, be sure to match the existing stick as close as possible as far as brand, speed, and timings. Dual channel memory is very fussy about the 2 sticks matching exactly. Sometimes even down to the batch number. That is why dual channel memory is always sold in matched pairs.
Btw, if you only had a single stick of RAM, you were running in single channel mode. That is about 10% slower than dual channel.