my LO did the same at his age but she didn’t sit up until closer to 7 months
You can let them sit for short periods with support if they have good head control, and can sit straight, without slouching. If they slouch over, they're not ready. I'd say we started around 5ish months.
Not too early, but only do it for a couple mins at a time. My son started sitting completely unassisted just before he turned 5 months old.
I'd say it'd a tad early. They need to build the muscles. Mine started sitting on unassisted by 6 months.