first bike for (only just) 4-year old? size, price etc

[todaywasfun]

DS wants a bike for his 4th birthday. He is average size for his age, not big not tiny. Am I right in thinking that I need a 14-inch one? 12 and 16-inch also seem to cover this age...
Tesco have a red one on promotion this week for £39-ish - is this a good deal?
Or maybe you could recommend one to me?
Not too heavy, we live on a moderate hill and I can see myself carrying it back up a lot...
Thanks!

DS of the same age had the 14" one. There was a chart to be measured against in Halfords but nothing beats sitting them on the bike as he clearly needed the next size up from the one recommended. Halfords also build it for you, although tbh I thought bikes came ready-assembled anyway.

He had this one for £50, reduced from £80, with a instore promotion via leaflet for a £10 voucher and £20 to spend on cycle accessories which made it a bargain.