K.,
Does your four month old spend anytime at all in the daytime with Daddy or when Daddy comes home from work? It seems like your baby does not connect with Daddy because he/she does not know Daddy.
But, if I am wrong, then I do not know what to tell you. Do you point to Daddy and say "Daddy loves you ..." or point to Daddy and say "...this is Daddy and he loves you!"
Does Daddy ever bottle feed your four month old? How about bathing or playing with your four month old. Do you lay your four month old with back on your lap and exercise his/her legs and arms to get ready for tummy time and crawling? If you do, let Daddy do this as well. This can be a great bonding moment for the both of them. Also, singing and say your babies name in songform is a fun and great way for Daddy to bond.
Hope this has helped you.... :- )