Kittens under 4 weeks rely heavily on their mum. If the kittens are warm and look healthy, mum likely hasn't abandoned them and is probably nearby if she isn't with them now.
If you haven't seen mum, ensure the kittens are kept somewhere safe nearby, away from people as they could spook mum and deter her from coming back. If the kittens are still there after 4-6 hours and meowing constantly (which often means they're hungry) then mum cat may have abandoned them.
If the kittens are cold to touch, sick looking or mum cat has abandoned them, the kittens will need immediate help or they could pass away.
If the kittens have been abandoned by their mum, our neonatal kitten guide may help you whilst you're trying to find somewhere for them to go.