should i be concerned

Imo and working in a baby room sitting unaided at 4 months is early. It's generally much closer to 6+ months. My little boy is 6 months and is starting to sit unaided for a few seconds before over reaching and toppling sideways. Is your baby rolling? Holding their head up? can they sit if supported? Are they generally happy, smiley and chatty? If they are doing those things, then I wouldn't worry, babies develop in their own time. It's so hard not to compare to other babies though. This is my third and I often catch myself thinking at this age the others could do this or that, I just try and remember he's his one person and by the time he reaches pre-school/school no one is going to know who rolled early, who walked late etc.

Every baby is different, some take their time to do stuff while others don’t, but if you’re worried then there is no problem to go and call the doctors or health visitor

One month ahead (baby turns 7mo next week). He’s only just started sitting unaided and he’s still a little wobbly sometimes.