In my own experience, as I was pregnant with my 2nd baby, my 4 year old son was very excited since he doesn't have playmate at home. He always hugs my tummy and keeps on saying I love you. As of now, my 2nd baby will be turning 5 months old already and my son's love on her did not fade. He loves to play with her sister and I don't see any problem with it. But, there are times that I can feel that he is somehow jealous at times and want to be the center of attention. He will just cry at the corner, I will know already that he also wants to be played as well. Its just normal though for toddler since they also needs special attention from their parents. I also don't let him feel that he is being left behind and as times like this like if he's feeling isolated, I will carry him and sing him a song then always reminds him that I love him and will forever be my baby boy.