Our older lad (5.5 years) is well happy on his geared Isla Beinn 20 (Small).
His younger brother (just about to turn 4), and has outgrown is Isla CNOC 14 (single speed) and is really desperate to go even faster and also get up hills like his geared older bro.
The next Isla up (CNOC 16) isn't geared (he's currently too small for a Beinn 20), so was after any non-Isla first hand recommendations for a good quality geared bike with smaller frame than the Beinn 20.
We can then go looking (new or 2nd hand) with some pre-knowledge.
In reply to ChrisJD: consider possibly just changing the freewheel on the back to a smaller one so that you change the gearing? Not sure if you can do the same on the front, I'll have to have a look when I get home.
In reply to ChrisJD: of course the front would have to go a size bigger if its changeable. It may be the gap he needs before jumping on the Beinn.
I've just had a thought, it won't help you but it might help me and anyone else who cares to tinker. My son has a 16" Cnoc, I also have a Brompton which has a 16" wheel, I wonder whether I could convert the Islabike to a 3 speed bike!
In all honesty, I would be tempted to get a reasonably cheap second hand 'run around/hack' bike for the short time he needs to jump on the bigger bike. Also have a look at the Beinn because whith the seat dropped fully, and the stem dropped fully, the bike becomes pretty low down, albeit quite long.
My older one got the Beinn at just about 5, and it was a bit big and heavy for him to handle at that point (of course, no way was he going back....) - so I suspect it might be a bit much for a just 4 year old, unless particularly tall. But the OP can easily test that I guess, given he's got one!
Equally, the cheap bike he had at grandparents led to a big crash because he couldn't reach the brake levers (did I mention brakes before? Hmm), though I think we managed to adjust them once we realised. I can't for the life of me remember if it had gears, though, quite possibly not.
In reply to Carolyn: Off topic (a bit) but how do you bridge the gap to a pedal bike? My boy is confident on his balance bike and will soon outgrow being on my bike. I was considering a tagalong but they are quite pricy if only used for a few months. He is a reasonable sized 3 and 3 months old but I don't even know if he will fit a tagalong yet
In reply to Stig:
trailgators are designed for 12 inch wheels up IIRC. However Seren was on her Honey bee (halfords) bike from about her 4th birthday apx 95 cm tall cant remember for definate but she had outgrown the bike seat
I think ours is great except that on her current bike the u bolts wont grip the tube n the frame properly so keep having to right the bike and it seems on a permanent angle bourne out by tyre wear.
Might have to be wary if they get lazy through using it though as we use it every day for the school run.
In reply to Stig: get a pedal bike but remove the pedals until he is used to coasting on it. Then add the pedals back on and 'teach' the pedalling part. It's easiest for him to have a coasting start. Then from stand still. Balance and steering are far more important then pedalling on a bike. Pedalling is just adding power. In my experience of cycle instructing, stabilisers and tag alongs make children lazy. They have their uses but not in the process of learning to ride.
His older brother already has the Beinn so know the sizing on that.
Changing the rear or front ring on the 14 is an interesting idea, but then ups become harder - he just needs gears..
Stig: both our boys went straight to pedalling (from being on balance bikes) at around 3y3m. Just go with it and no tag alongs, taking pedals off, or dreaded stabilisers! They will get the pedalling bit easily, its the initial starting off that is the harder thing to learn.