I personally think that 4gb of ram is more than likely tons of Ram for the usage that you are describing ibis99. I have a 2010 imac with 4gb of ram as well and I do a lot of audio recording in garageband and logic express and I am still ok with 4gb of ram. I highly doubt that web, email, movie watching, music listening and web design will breach that 4gb unless you are doing all at the same time!

. Web design software normally doesn't tax the system too much at all (rapidweaver, flux, iweb, etc...).
GGJstudios is absolutely right in his recommendations to check page out and swap used. If both 0 then you haven't exceeded the 4gb at all to my understanding.
Here is a test that you can do:
First go to your applications folder, then utilities, then activity monitor, then click on the system memory tab at the bottom of the screen.
Now in that section you will see the following Ram types and what they mean are as follows:
Free = Free Ram, nothing is using it or holding it for use
Wired = Ram used by Osx, cannot free up the operating system is using it
Active = Ram used currently by all other apps/programs
Inactive = Ram that was "just used moments ago" by all other apps/programs, the system stores it here so that if you open it again soon it will open faster as it is still in Ram (good mac memory usage

).
Used = Simply the total of Wired, Active, Inactive.
So for example, my system is showing:
Free = 2.31GB (free for anything else I'd like to open)
Wired = 663.5 used by OsX
Active = 473.3 used by other apps/programs
Inactive = 595.1 used by recently opened apps/programs (if new programs need it, they will take from free, then inactive, so inactive is essentially free as well in a way, see what i mean?)
Used = 1.68 (LOL, note my numbers above keep changing all the time and I didn't want to pull out a calculator, pretend this adds up to wired, active, inactive

).
Bottom line, for the use you are describing I wouldn't waste the money on a ram upgrade if your current ram isn't even being fully utilized based on your needs. It would just be a waste of money.
Hope this helped...
Peace...