4 GB is usually enough for most cases unless you're starting tons of stuff or running something that takes a huge amount of memory like video editing. With the 64 bit version of Windows 7 you have you can add as much RAM as your hardware will allow (usually 8 to 16 GB) if that's your bottleneck.
One other place to check is what is being started automagically when you boot up. Run MSCONFIG and click the Startup tab to see all the tasks that're being started when you boot up. A whole lot of applications assume you want to have parts of them resident all the time when they really don't have to be. Examples are Adobe, Office, Quicken, computer manufacturer's stuff (Toshiba wants to load 9 things on my laptop), etc. To run MSCONFIG click Start and type MSCONFIG in the search box. Then when you see MSCONFIG.EXE in the programs list, click on it. Once started, click the Startup tab and uncheck everything you see there that you know you don't need. Be careful not to remove your antivirus software.
I hope this helps. Good luck.