Firstly, I had Lion then

To answer your question - when I had Photoshop, Indesign, Bridge, iTunes, Safari, Word opened and servers connected it had gotten very slow sometimes. One time it was Safari being unresponsive (when going through tabs), the other time it was InDesign "beach balling".
The RAM was being maxed or almost maxed out in Activity Monitor. It was on rare occasions I must admit - mostly because of Safari or huge files handled by InDesign. Nevertheless getting 8GB wasn't pricey, so I didn't hesitate and upgraded.
This was especially bothering on the 13" MBP (lack of dGPU?), but I went with upgrading both 13" and 15". I sold the 2 x 4GB I had left - for a symbolic price, but better than nothing.
Obviously it all depends on what you need your Mac for. If you handle big Photoshop (100MB+) or InDesign files then I would suggest getting 8GB for a comfortable work.
And like I said, if OP is going to use iTunes, Safari and such then 4GB will probably be enough.