Armin answers: The first thing you need to do is not let your guilt get out of hand. A little bit of guilt is okay, but some fathers (and mothers)--in an effort to make themselves feel better about not being able to spend enough time with their children--end up withdrawing from their kids emotionally. Leaving your wife to take care of the baby is a habit you don't want to get into (and if you notice yourself doing this, there's still time to stop). The earlier you and your baby start getting to know one another, the closer and better your relationship will be.
Another common trap parents sometimes fall into is trying hard to make up for lost time. You might, for example, attempt to cram as much active, physical father/baby interaction as you can into the few hours you do have together in the evenings after work. While all that activity might make you feel a little better about being away from your baby during the day, you'll also end up overstimulating him. So before you start tickling and wrestling and playing with the baby, spend a few minutes reading or cuddling with him, quietly getting to know each other again. At four months, a day away from you is a long time for your baby. You'll both feel a lot better if you spend a little quiet time reconnecting.
While there's no practical way for you to make up for the time you're spending away from your child, it's important that you find some middle ground. Separate work time from time with your child. Make sure that whenever you're with the baby, you're with him 100 percent. Forget the phone, the computer, the newspapers, or the TV. You can do all those things after the baby goes to sleep, before he wakes up, or while he's busy nursing. You also might want to explore some different scheduling options for your office: getting into work an hour or two early might give you and your baby a few relaxed hours together in the afternoons. And telecommuting to work one day a week allows you to spend your commute time reading your son a book instead of sitting in traffic.