4 month old question

Britta
My baby girl is now 4 months old. She will kick things if she is laying down, but when I try to hold her and get her to stand and put her feet down, her knees just buckle and she won't put pressure on them. Is this something to worry about at this stage? She also hasn't rolled over either way- only get herself to her side when she's on her back. Thanks in advance!