If you decided to extended rear face your child, when did you finally flip them?

My DS will be 4 in a few weeks and is still rear facing. I see so many recommendations to rear face as long as possible until your child is over the rear facing height/weight limits on your seat. However, my seat goes until 40 lbs or 40 inches tall... my little guy is only 26 lbs and 35-36ish inches tall and could probably rear face until at least 5.

I’m considering flipping him when he turns 4... but he’s still completely content rear facing and has never asked to switch.