4 Year Old Scared During Swimming Lessons

My experience was that my children did better when I was not there. Well, I was really there, but around the corner where they could not see me. After a few weeks, I started "being there" again and he was fine.
When he was younger I was in the water as well. Might be worth a try?
I also encourage play at home that lessens water fear in the bath/shower. Blowing bubbles, pouring water over their heads (make it a game).
I recommend that you continue with lessons, private or group. This will help the kids when they are around pools to some extent, but there still has to be adequate adult supervision (this is just a reminder for everyone).
As to why he was fine and then not fine, what phase of the moon is it? ;-) Seriously, it's hard to figure out what is going on in their heads. No ideas.

I had the same situation with one one of my sons, I believe in swim lessons. So my children started around 2, my now 7 year old. Was always unwilling, and not real confident. We never forced it, just always included him in the class. I never let him move up to the next level, untill I felt he had truely passed. Once he was 5, all of a sudden he developed the confidence to swim. He always had the same instructor each year, and I sat at the complete opposite end of the pool. Just continue with the classed. Once he feels he can do it, he'll catch on. It's very important he knows how to swim. Private lessons may help. But you never know.

I would switch to the private lessons...esp. if it is with the same instructor that he has had the past two lessons this week. I think switching around to too many instuctors or forcing him to participate in a class setting may breed insecurity and only fuel the fear. I think the instructor needs to be patient and allow your son to go at his own pace, but I don't think that taking him out for a few years is a good idea. I think it is the easy way out.

I think private lessons are worth a try. It can be a really different experience working one-on-one with a teacher than in a class. He may feel more secure knowing the teacher is completely focused on him.