Depends on the device you are using,
http://www.hulu.com/support/article/197541 so if your device is 1080p it will require more bandwith.
Minimum require is around 1.5Mbps for smooth playback on SD and around 3Mbps for HD. 4Mb just doesn't sustain enough for the HD setting, when it does burst it changes to HD andthen it will start buffering again shortly after. Just force it to keep at SD and you're fine.
Netflix has a slightly different option, where you can pick your quality based on how much bandwith you can use per hour. Their highest quality is around 2.3Gb per hour (if i remember) and that is 1080p "SuperHD" with 5.1 digital audio. It looks amazeballs, and I prefer watching on NetFlix than downloading off newsgroups when I can.