I know two people that are/ were on prednisone and both of them had greatly increased appetites while on the drug. I may be wrong, but it is my understanding that the medication itself does not cause weight gain (other than fluid retention) but the increased hunger does. However, the drug can cause a redistribution of fat that will alter your appearance. If you have the will power to withstand the urge to eat for a month or if you can counteract the extra calorie consumption with exercise, you will be fine. Also, everthing I have read on the topic lists weight gain as a side effect of long term use of prednisone. I don't know if 28 days would be considered long term.
Several sites stated that the effects are reversible if the dosage is 10mg or less but you are above that level.