Had the same dilema back in April this year when our daughter turned 4. She'd been riding a balance bike successfully for 3 months beforehand. Isla advised to go with the 14 which we did, and she took to very quickly.
However, we've now bought the 16 for her primarily because it was impossible to find knobbly tyres for the 14, and most of her crashes were as a result of no grip off-road with the tyres the bike came with. The 16 can be specced with Conti Explorers (just like Daddy's) and are a real improvement. It also has f & r brakes whereas the 14 has just rear. Pedals are bigger too on the 16, otherwise little else different spec-wise. Sizewise, it's not much bigger, but I guess things are relative when you're that small!
Sizewise the 14 still fits her, and back in April we were glad we'd gone with the smaller one, but it was amazing the regularity I was raising the saddle. Now, the 16 is definately the better fit.
I guess the final decision comes down to cost – from our perspective, her brother is only 18 months behind her, so the bikes should both be well used, and they keep good 2nd hand value. If cost was an issue though, I'd probably go with the 16 as I suspect you'd end up getting more use out of it.
Final thought – can he ride a balance bike? If so, then the 16 shouldn' be too much of a problem, but if not then it might be a bit of a handful. The bikes are not esp light (again relative to a 3-4 year old) and picking one up after the inevitable crash can be hard work.
Hope this helps!
PS They are brilliant bikes – my 4yr old rode about 6 miles last week along a disused railway line on the Welsh coast – a very proud Dad moment!