L.- it depends if you have asked him to stop looking at adult websites. If you have discussed with him that it hurts you that he is looking at them. I have been with my husband for over 15 years and I'm ok with him looking at adult websites- sometimes it's for inspiration for us to experiment. I find this healthy for our relationship and keeps things fresh. Of course, I have never expressed to him that I do not want him to do this, I personally don't have any issues with it, this may be different in your case. In our 15 years, we have never cheated on each other, nor wanted to be with another person. He is an excellent father to our 23 month old and wonderful and caring husband to me.
If you find that his perusing the internet keeps your intimate life active, then I think it's a healthy thing. If you find it disturbing, then you two should have an open discussion of why he looks. Maybe he should include you and it could be fun. You say he is a good father, but you don't mention if he has treated you well for the past 4 years, other than this issue. I think it's reasonable not to feel sexy after birth (Stretch mark, gaining weight), but is that just you or is he saying things to you that also reinforce this? I also didn't feel so hot after the birth, but perhaps join a mom's walking group, exercise releases chemicals in your body and will make you feel better about yourself too.
I hope that at least you two will discuss the internet issue since it is making you feel uncomfortable and I hope that it works out for you and the baby.