My DS is 4 months old and outgrowing his baby seat. I would like to get a 0-4 years seat, but prices vary hugely, from around £60 to £300 plus. We don't have huge amounts of money and aren't fussy about 'labels' but safety is paramount. Am I compromising on safety by going for a cheaper seat?
Are you sure he is growing out of it? My DS was a big baby and he was in his until 9 months. 4 months sounds very young to be out of it. You can take the head hugger out, and usually move the straps up. The Maxi Cosi Cabrio Fix has a wedge of foam under the cover to bulk it out for babies too, which you can take out to make it bigger; maybe other makes and models do too.
You don't necessarily need to buy the most expensive seat, but I would avoid the very cheapest as they tend to be flimsy. Mothercare own brand for example are good quality, but about 1/2-2/3 the price of the big brands. The nania/babystart (+ various other obscure names which they keep changing) ones are particularly bad. Graco is also another make which isn't as expensive as some but are okay quality. I don't think mothercare or Graco do a 0+1 seat though. I think the Britax First Class at £99 (or at least that's how much the one I had cost) is about the cheapest I'd go for this kind of seat.
But agree with the others that it's unlikely he's outgrown it at 4 months. DS fitted in his until 18 months. If he's under 13kg and her head hasn't reached the top yet then he'll be fine for ages yet.
However if money is no object to safety I would avoid the group 0+1 seats and keep him in his infant seat as long as you can, then move to an extended rear facing group 1 seat as these are the safest.
wish i had known about it before he was born as it lasts from 0-25kgs and i would have saved myself the baby carrier money, as i couldn't lift it anyway! but as others have said the baby seat should do to 10-13kgs. ds is 10kgs at 6 months and still has loads of room left in baby seat.