My son got the Doc band when he was 7 months and wore it for about 3.5 months. His H. was really flat in the back. We could have used another treatment, but after the first treatment, I felt like his H. was round enough to be covered by his hair. Now, only I sometimes notice the slight flatness in the back when his hair is wet, but no one else notices.
Like other moms mentioned, there was no adjustment period for my son to get used to the helmet. He never knew it was there. The only "drawback" was that he found he could bang his H. against stuff and not have it hurt. So, when the helmet came off, he was in for a rather unpleasant surprise when banging his H. on stuff actually hurt! Some people made weird comments (like,"Does he knock his H. around a lot?"), but nothing major. Mostly, we were approached by people talking to us about the experiences they'd had with their kids' helmets.
At 4 months, though, you might want to give it a little more time to see if his H. rounds out a bit. For us, we began consulting with Cranial Tech at around 4 months and decided to go through with treatment after 3 months of repositioning, increased tummy time, etc. with no results. Some kids do improve, though, so if you can avoid the costs of treatment, it'll be worth a slight wait.
That's another thing...our insurance covered absolutely zero of the cost. We paid the balance in full up front, so we got $300 off, but still, $2,700 is a lot to pay for some rounding of the H.! We don't regret the cost, but we sure wished at the time that we didn't have to do it!
As for decorating, Cranial Tech should have some names/numbers of people you can contact if you don't feel comfortable decorating it yourself. I'm not very artistic, and I wanted it decorated, so I went through one of the people Cranial Tech recommended. I think it was around $50. I dropped the helmet off, and it was finished about two hours later. I think he was out of his helmet for a total of about three hours (I could be remembering that incorrectly, though). You're not supposed to have them out for that long, but he was fine. We chose a "tough guy" design--black with yellow/orange flames and his name written in red across the front. I even got him a matching onesie at Hot Topic, so it was really cute! The woman gave me a list of paints to buy for touch-ups. We used acryllic paints in opaque colors and Elmer's "Painters" paint markers. We used a Krylon clear glaze to help protect the paint job.
Hope that helps! Good luck with everything, and send me a message if you need any more info.