Well, let's dissect this... You are either imagining the problem, or the problem is real.
If you are imaging the problem, then there is no "real" problem. If the problem truly exists, then it is most likely caused by extra CPU cycles being used, and NOT by the physical presence of more RAM.
If you are running one of the flavors of Windows Vista 32-bit:
The jump from 3GB to 4GB does pretty much nothing, since your OS can't use the extra 1GB of RAM. Therefore, the problem is either imagined, or caused by something else (such as, you installed programs since you got your laptop, those programs use CPU cycles when your comp is idle... more CPU cycles used means more heat).
If you are running Windows Vista 64-bit: The OS will "see" the extra 1GB that you installed. The "installed programs" scenario described above is possible, where programs use extra CPU cycles. But Vista may also be doing some SuperFetch activities. SuperFetch basically takes the extra RAM that you have, and pre-loads it with the data required by commonly used apps (eg, if you use Firefox a lot, it will pre-load Firefox into RAM). More RAM means more SuperFetch activities, which means more CPU cycles, more loading into RAM, and more reading from hard disk.
Whatever the case may be, I would just keep an eye on the overall system heat and battery life. As long as those are within spec, then anything that happens shouldn't really be a problem.
If you do want to pursue ways to reduce heat and extend battery life, you may want to do a search for Undervolting using RMClock, and undervolt your CPU. Undervolting simply reduces the voltage supplied to your CPU running at its current clock speed. Less voltage means less heat, and longer battery life.