Ds1 4 months
Ds2 6 months
Ds3 introduced at 6 months no real solids till 8.5 months
Dd1 Introduced at 6-7 months no real solids till 9 months

I (personally) think wait till 6 months and use food as an activity rather then a food iykwim? Then if they take to it then keep going otherwise wait till they are totally ready..

All my kids are pretty good eaters and eat a large variety of foods.

At 4 months they tend to start noticing what we are doing and mimic our movements so this is what your LO might be doing