You should go ahead and try it! It wont hurt anything at all.
In the end, you'll have an extra 2Gb's of memory.
Only you will be able to tell if it's slower or not.
My bet is that even if it is a bit slower, you wont even be able to tell, without running a benchmark!
I say do it!
Heck, if you lived in the US, I'd send you one of my extra 6Gb kits!
However my system wont boot with the extra 2gb stock in..so its a no go for me!