There is nothing wrong with spin seats, but they are a smaller size.
When they say "Up to 4 years" this is like clothing sizes. So children outgrow these seats roughly when they move into age 4-5 clothing.
To compare, your LP is currently 9mo but he is wearing age 12-18 month clothing. So you can see that he's a little ahead of those expected age ranges for things, and he might go into age 4-5 clothes when he's still 3, for example.
It might be that this growth slows down as he gets more mobile, or he's recently had a growth spurt and will plateau for a bit, but if he is on a higher growth centile, and you buy a spin seat "up to 4 years" (or the ones that go up to 12, the rear facing/5 point harness mode only goes up to age 4 approx as well) then you will find that he is only three, or sometimes not even three yet, when he outgrows it.
Most people after the "up to age 4" seat move children into a high backed booster type seat, but this is not appropriate for a 2 or 3 year old, because it just uses the adult seatbelt and doesn't have a 5 point harness. Toddlers are too little to have just a seatbelt, they play with it, they get their arms out and they aren't sensible.
So, no problem with buying a spin seat now. It will be lovely for your little boy right now. But they cost £200-300, and you might find in a year or two you are going "Help, I need a bigger seat with a 5 point harness!" and then when you find it, and fork out another £200 or £300 for it you'll say "Why didn't I just get this one in the first place?!"
So this is us (older mums been there done that) saying don't do that, just get the bigger seat in the first place 