Boy, you guys are getting close!
Try 4h15 first. Better to go slow. And I'd leave him there for 2-3 days before trying to extend again. You guys are REALLY close to 1 nap nirvana! Also, it can take a week or more for them to take longer than an hour nap once you *do* move to 1 nap. As you can imagine, their body expects a 2nd nap since that's what they've had their whole little life! So it takes adjustment time. I was so scared my guy would never sleep more than 1h/1h10! But sure enough after a week he started sleeping 1.5, and then another month after that we started getting 2h (I think he just needed some more time to grow into his A time). Now at age 3 it isn't uncommon for him to sleep 2.5 or even 3h! So it *does* get better.
Yes, I would have stretched him because he woke so late a 2nd nap was going to be impossible. So, better to make that one and only nap as late as possible just in case he short-napped because then bedtime isn't TOO far away.