How small is quite small? Pic? They are a dwarf type anyway, so.....it might be on track, but hard to tell without more info. My current Tiny Tim never got more than a foot tall and is producing consistently.

Looks good imo. Some might suggest removing those flowers, but I wouldn't. Looks right on track to me. It will bush out more, but probably only grow maybe another 6" or so. Nice plant, let it grow.

It is ~5cm tall. Is that actually large enough to produce fruit?

5cm is about 2". It looks taller than 2", but maybe the viewing angle. I like the idea of raising it up in the pot too. Either add soil on top, or raise it up by putting soil underneath. It's a dwarf, and will produce just fine, not to worry.

It is around 4” tall. Is that large enough?

Yes. It will only get to 10" or 12" tall as it is a dwarf.

https://www.victoryseeds.com/tomato_tiny-tim.html

Tomatoes are usually smart enough to know when it is time to bear fruit. That's about maturity, and not necessarily size. If the plant can support fruit, the flowers will fertilize. If not, then they'll drop off. But realize that once it starts fruiting, it isn't going to grow a lot. Four inches is kinda small for a 6-week old plant, unless the temps have been low.