What Daniel said!
On my 4wt in particular, I have tried 3wt line and yes, it still threw a fly line just fine, I found the results to be less than I had expected. When going to 5wt line on a 4 wt rod (Winston BIIt), again, it cast and for making short casts, it worked fine, it was less good when trying to cast longer distances.
When all is said and done, I have found it best to just stick with the number printedon the side of the rod.
The most important thing is to get out there and enjoy a day of fishing (and if you're lucky, enjoying some catching to go along with the fishing).