I'm a lifeguard and used to give swimming lessons to children. I think the courses were going on for 12 weeks (maybe 10 though) and it was twice a week, for an hour each. Most kids only needed one course, some had to take it twice if maybe they took longer to get comfortable in the water, some are really scared at first.
But, the children we taught were usually around the age of 5/6. So for one I think 4 is quite young to learn to swim (actual breast and backstroke is what we taught and when we were done children were able to do both for at least 25 meters without a break). But not impossible at 4!
But the second thing is, once a week for half an hour is I think not often and not long enough. They have to practice the moves and for every practice, we had to kind of start over comparing to last lesson. Obviously not completely but they do forget a little bit from one lesson to the next and moving in water is so different from their usual movements, they kind of need a bit to get used to it again during the lesson, especially since usually they can't practice in between lessons. So i think it's not surprising that with the lessons spaced so far apart and being only 30mins long he hasn't learned too much yet. Definitely does not mean your son isn't talented or won't ever learn to swim!
Is there any other swimming lessons? Or, you could go for another time during the week and practice with hin yourself? (It's best to not use too many helpers, most effective is holding the child with one hand on their chest and practice their arm and leg strokes, while moving so the child knows what it feels like to move with the strokes and gets the feeling he's moving not because you are but because of him doing the strokes. The stronger the child becomes and the more secure it feels during practice you can start holding it less and less until you barely touch the chest anymore and eventually the child can go longer and longer without any assistance.)
I hope all of that made sense, i'm not a native speaker and that was kind of hard to explain

But ultimately, once a week for half an hour is not enough, especially since 4 years is quite young, in my experience at least.