actually, i would treat the 3 and 4 year old the same.... keeping in mind that expectations for the 3 year old are slightly lower, but not really by a whole lot! don't let yourself fall into the trap of dismissing the 3 year old's tantrums as "just being a typical three year old," while coming down hard on the 4 year old for the same kind of tantrum, because the 4 year old will pick up on the inconsistency and double standard and he will be resentful of it (which could actually cause even more tantrums and discipline issues). our kids are aware of a lot more than we give them credit for!
your sons are close enough in age that they will benefit a lot more if you are consistent with the discipline for both kids and if you'll have the same expectations for both as well. it will help your 4 year old feel more secure in his place in the family, and it will help your 3 year old become more independent in wanting to be "big" like his brother.
i use the same method with my two girls and they are exactly two years apart (ages 4 and 2). since i started doing this, i've found that their behavioral problems have dramatically lessened in the past several months! good luck!
ps - my 4 year old looks like she could be 6 years old, so i completely understand the looks you get from others when a tantrum happens out in public!