Can you be fired for calling in sick 4 times in over a year?

I have been working as a courtesy clerk (bagger) at a grocery store for about a year and 2 or 3 months now. Recently, I called in sick for the fourth time ever. My manager told me that I call in too much and that its "always on shifts where she really needs me". She told me if I didn't show up or get the shift covered, she would take me off the schedule permanently. I wasn't able to get the shift covered, and felt too ill to go to work that day. I feel this is unreasonable as I am never late, have never had consecutive sick days, and am generally a good employee. Some of my friends who work with me used to call in about once or twice a month, have had no call, no shows, and are frequently late, yet they still have thier jobs and were getting more hours than I was while working there. My main question is if this seems reasonable, and if not, what should I do about it?