You can wash and dry the part you've got finished, but even with a wool yarn, that may not counteract the curl at the lower edge. Four rows of garter stitch is only 2 ridges, and you probably need a bit more, like 6 rows/3 ridges or even 4 ridges. It tends to want to flip up really, more than curl, and you can do a row of k1 p1 at the transition which could help.
This article can help.