Watch the Four Note Chords online guitar lesson by Stu Hamm from Stu Hamm U: Solo Bass
When playing four note chords it will greatly benefit you to learn to fret all four strings with one finger. This is a very guitar oriented technique, but sometimes you have to do what you have to do. If your hand cramps up or starts to hurt, stop for a while and stretch your hand out. as with anything, you can't learn it all at once, it will take repetition and patience.