Only trying the larger amount of memory will tell whether it improves speed with the configuration and applications you use. But a single 4GB DIMM will not be as fast as it was if you originally had a pair of 2GB DIMMs, since you will no longer have dual channel memory operation.
Unless you have benchmarks, telling whether a system is matching the performance it showed when new is very subjective. A new computer usually feels fast in comparison to the the one you had before, after a while you get used to it and you no longer feel the speed. But it's also likely that over time software has become more demanding (e.g. the anti-virus or internet security software), also the hard drive fills up, thus utilizing the slower sections of the drive, and if defrag is not run periodically the file system becomes fragmented. Hibernating the system over substantial periods rather than shutting down and restarting next time can also result in a sluggish system, since the computer has effectively run non-stop for its total on-time since the original hibernation, and boot-file optimization never occurs.
If a system seems slow it's also worth double checking for any type of malware - even something that may not be directly harmful (often called a PUP, Potentially Unwanted Program) can have the nuisance result of dragging down system performance.
Edited by Platypus, 18 March 2013 - 02:19 AM.