yes it's no problem if ur baby wants to sit. my baby also started trying to sit when she was 4 months. babies only do things which they r capable of. so don't worry. my baby sits on her own now and she is 6 months. but put a lot of support and never leave her alone. making her standing should be avoided though.

If baby is comfortable then there is no problem. However make sure that he is sitting with support. In terms of standing if there is additional pressure on the legs it can lead to bow legs in the future and hence avoid till baby is able to stand on his own.