Safari &/or Firefox & flash could certainly be eating up 4GB of RAM. I often find my free memory at less than 100MB - have seen it as low as 28MB free and with no flash content being used.
Have had to start using a memory manager with 10.8 to free RAM instead of restarting or quitting Safari & Firefox. Not at a Mac currently and can't remember the name of the one I'm using.
edit:
But, before you even go there, as mrplow states, you need to open up Activity Monitor and find out whether your issue is related to memory or CPU intensive tasks.
Do you mean speed up your Mac start-up? Well, I know Magican can list all your start-up items, you could check them and close some rarely used items, hopes that helps~
There is
absolutely no need for any 3rd party application to find your startup items.
You open System Preferences - Users & Accounts and look at your log in items to delete, add, etc.