I certainly can't comment personally about it since I have a hard time with these types. I can comment on a grower in CA that grows them in every shape and fashion you can imagine - trees, topiary, and has one tuber that is at least 8 inches to 12 inches across. I don't know if I would start pinching at 4 inches long though. Maybe give it a foot or more unless you want to try your hand in propagating from the pinched tips.

Here is a picture of one of the grower's monstrous boliviensis type begonias (Bonfire).

Hope you can see this on FB.

https://www.facebook.com/photo.php?fbid=635097569892286&set=pcb.633210840061913&type=3&theater

Well, thank you for replying...I was beginning to think noone was looking. I haven't pinched (but still want to)..the first blossoms are open now and it looks good for such a young plant. Very vibrant red.

This site is almost dead. I will buy a couple of boliviensis hybrids and usually lose them late summer if not before. Guess it is too hot here for them.

Our gardening centers get them in each year looking like a million bucks but they go downhill before fall.

Here is one I had a few years ago - did well for a few months so I know if I buy them I may as well treat them as annuals.

Well that's interesting...I know of greenhouses in the area that have carried them over for years. This is my first try with them, normal tuberous begonias behave like you describe for me, but boliviensis is reputed to be able to cope happily with the heat. That's why I got it...of course it's experimental for me too.

One of our club members was able to grow them from little cuttings but then he had access to a greenhouse. I never had any luck propagating them. They must be easy since the nurseries have oodles of them in spring.

I tried Sparks Will Fly last year and it was worse than Bonfire and boliviensis in our heat. Maybe went two months, three tops.

I'm about to pinch I think...because it's growing SO fast. Forgive the lazy sucky camera phone photo...but just to give you an idea. (note the plant looks much healthier than it appears in the camera phone shot)

Pinch away...

Pinched the three longest stems...will let the other three grow a few more days before they get a pinch too.