I've worked FT with children for the last 6+ years, but I've employed a number of people who've worked PT - commercial property in a firm with very little corporate support and few American clients.
4 days works with some determination; 3 days is too stressful for words. I'd avoid Fridays as the day off.
The determination is both from you and your employer. Really it's up to you how much you check your Blackberry on your day off. You have to accept, I think, that there are times when your day off is not really very much of a day off at all, just as some weekends are not really time off. Are you planning to work PT for a while while baby is small, or truly have you never been interested in the greasy pole of law firm promotion, and if this works will be perfectly happy as a senior assistant?
In terms of the employer, we try to make sure the PT lawyer has a good FT secretary, who can exercise sensible judgement about when to call your mobile and say "you really need to look at the e mail from X which has just come in". People also find it helpful to have a nominated trainee who will deal with their files on their days off - this is great experience for the trainee and makes the PT feel they are helping someone by dumping active files on them, as opposed to asking favours every week. Obviously it needs to be a halfway competent trainee...
We took on someone from a top 15 firm who had been working 4 days a week for 5 years before her major client even noticed. She did a bit of work on her day off but not much. We've also taken on someone who's been working 4 days a week (apparently) for 15 years, but in fact she works far too much on her day off and IMO would be more accurately described as FT with one day from home.
The solicitor who found it hardest to work PT for me was the most junior one - she went on mat leave at 6 months PQE. I think that's a reflection on her seniority (and to a certain extent personal circumstances) rather than on her, because she was a pretty good lawyer. She's left the law for teaching now.
Baby's father does need to do some childcare from time to time if PT working is to work for you. Is he up for this? And you need some form of back up plan for childcare at short notice in emergency. I have a nanny; one of my partners in employment law who works 4 days is able to call on her mum.
It is fair to say that a lot depends on your client base and the partners you work for. Are you going to be the first PT in your group? When DS1 was born I was the first mum in a fee earning role in the London office so FT was enough ground breaking for me, but since then we've had a range of people work PT here, and the good ones have been promoted.
HTH