S.W.
hi K.,
i completely agree with most of the other posts about him wanting more attention, the newborn in the house, him not really understanding the meaning of the word, and you being his lifeline. i don't believe punishing him is the answer. i also don't think ignoring it is the solution either.
self reflection is not something 4 year olds excel at but if i were in your shoes i would sit him down (either just you and him or with your husband as well) and talk to him about the behavior. explain to him that this is hurtful to daddy and that you both need to find a better way for him to get attention. you could try perhaps a secret word between the 3 of you. almost like a password that he can use if he needs a moment or two of undivided attention from you or your husband.
best of luck with everything. :-) S.