What stage should my 4 months baby be at?

my little boy turned 4 months on monday. and this is what he is doing. he feeds himself his bottle and juice. he drinks his juice out of a beaker. he can sit up for 10 seconds unaided, he pushes himself backwards in his baby walker. and i put him on solids 1 weeks ago as he is a very hungry baby. and hes feeding himself with a spoon. is this normal for a 4 month old? is he behind or just right? are other 4 month olds doing more than this or less?