I wouldn't be. All babies do stuff in their own time. My sister(now 5yrs old) Never rolled, or crawled, or cruised. She went from laying, to sitting, to standing, to walking. Sometimes babies skip stuff.
no not at all! each kid is different and at 4 month they dont do much anyways ( its great ) lol j/k give her another month or two. you will see alot of change. with my son i let me play in the floor lot bc i figure if he was being held alot he would never learn how to roll over, craw, ect.... but at 4 months he hadn't rolled over either it was about 5 month then it was all over from there!!! lol