DS2 had a Britax Advansafix at that age (if I was buying again, I'd go for Evolvafix or Maxi Cosi Titan i-size).
If you're asking what the best car seat is for a 3-4 year old though it's a complicated question as what suits one might be totally unsuitable for another.
The important questions will be:
How tall/heavy is the child, and how close to 4? (This is important because there are two common sizes of seat for this age group, and you don't want them to outgrow too early)
How sensible and calm is the child? Some sit nicely and behave predictably whereas some are still quite wild and active.
Rear facing or forward facing? Rear facing is safer and more universal, but many people prefer forward facing by 3.
Has the child outgrown their current seat or is it e.g. needed for a sibling? What seat is this?
Do you have isofix and top tether points in your car?
Do you have a budget in mind?