I've developed two bags under my eyes after fat transfer. They look exactly like festoons. Is this normal?

What is described sounds like an undesirable deposit of fat, since most of the swelling is gone. You should have this evaluated by your plastic surgeon. One of the risks of fat transfer is putting too much fat in one place and getting a lump. The area described is soft, but tender at four weeks. The tenderness may be related still to the surgery. One would want to wait about three months to re-evaluate, if possible.  Most non-viable fat will go away by that time and you and your doctor can see if these soft, tender bags are going to persist.  If it turns out they do persist and are areas of over-fat grafting, further steps may be needed to remove them.

Swelling in the festive region can complicate the aesthetic convalescence from a facelift. Steroids are rarely indicated and it must be remembered that they can have negative consequences as well. As long as the trend is favorable I would leave the healing process alone.