Hi D.!
I used the Playtex Natural Flow bottles and I also found that the fast nipple was hard for my son to take. So I kept him on slow. There really is nothing wrong with keeping him on slow. It takes forever for them to finish their bottle but at least they don't gulp it down and still feel hungry. Once my son turned 9 mos, I switched him to the fast flow once again and he drank it down like a pro! I suppose he just wasn't ready when I first tried it at 6 mos.
Now he drinks mainly from the sippy cup and only gets a bottle at night before I brush his teeth and put him to bed. It's part of his "downtime" routine.
Anyway, I suppose if you are intent on making the switch to fast flow, you could expiriment with other bottles and see if their fast flows are any different. I know that many people use Dr Brown's or Avent. I breastfed and the Playtex Naturals were the nipples my son liked best. Also, when giving him the fast flow,try feeding him when he is propped up a little and not laying all the way down so that he can swallow better. (Imagine trying to take a sip of a drink while you're lying down).
4 mos is still very young and I'd be hesitant to make the switch to fast flow so soon. Perhaps you should wait until you start giving him solids and his swallowing techniques improve? Every Mom knows their own child best so if you feel he is ready for the switch and you're not happy with Playtex, try expirementing. As mentioned above,Avent and Dr Brown's are the most popular.
Good luck!