I am speaking as someone who sent my child to school part time for the whole of reception. It made zero negative impact on his education and on his social life and caused zero disruption according to 1) his teacher and 2) .
My son went part time three days a week until the term after he turned five (the summer term). He was able to do this with the agreement of the head because attendance statistics for those under five do not count towards the OFSTED required attendance statistics.
I was able to send him part time for the summer term also as it was the last year in which a child could be flexi-schooled and have it 1) marked as educated off site (so did not affect attendance statistics) and 2) it was mid term so did not affect funding. Again the head saw no problem with this, and nor did I. I wish that this were still in place, and regret the way flexi-schooling has been muscled out by the requirement that the child is now marked as absent.
It's true my son was not the only child to be exhausted at the end of a three or four day week. But that is not a reason to keep him in an exhausting system. If anything, it's a good reason to reform the system. I felt keeping him in school full time under these circumstances was counter productive. Also I wanted to spend time with him as I feel that four is ludicrously young to put a child into compulsory education. We didn't do school work on our days off; we just had fun, or had day trips or played or went to classes or hung around the house.
Your head might not mind if you point out that 1) you do not think that it is in your child's best interest to stay in school full time 2) you say you will send him full time the term after he turns five 3) offer to review the situation if the school feels he is 'falling behind' 4) point out it will not affect the school's attendance statistics and 5) say you will not advertise it in case they are worried lots of parents will want to do the same (in my experience, lots of parents won't as it is quite hard to be flexible with days if you are a working mother or father). Also 6) let the school choose the times that your son is absent (e.g. two afternoons a week rather than one school day, in case they do literacy and numeracy in the mornings).
I hope you get the outcome that you want.