My 4 month old weighs 18+lbs and has measured at the 97th% since 2 months? He just started solids, should we be concerned about his weight?

Always, but.... .. What about his height? If bothe height and weight are in the same percentile range, that is totally normal. If proportionanally overweight, as he gets older, you have to make he is not being overfed. By using feeding for a soother, for example.