mayb you want to invest on bumbo chair? cos now, at 4mth+ i think walker is abit too early. i let dear son sit walker when he was nearly 6mths old. even till now, i dont let him sit in it for very long time. as long as you limit the time, should be okay. but make sure you're with her when she is in walker.
as for high chair, depends on which one you get. if you're refering to ikea type, that one i think better wait till your girl is slightly older, mayb 5mth+ or even 6mth also.. but for those which can convert into 'rocker bed' and highchair, should be okay. cos dear son has been using that since 1st day at hm. from lying flat all the way to sitting up straight. can slowly adjust. =)