Desferal, IMO, is probably safer in the long run. You can look up the side effects and warning at these links:
http://www.rxlist.com/exjade-drug.htm
http://www.rxlist.com/desferal-drug.htm
Hopefully, they did not start her on a "full" dose right out of the gate. Your liver and kidneys, as you age, may not be able to clear the drugs as quickly as a younger person so you would have to reduce the dose.
I assume your mom's FE was normal before her diagnosis. If it was, then, a FE of 4000 should not pose a big problem at this time. It usually takes years for the iron to do damage. We've been told that the first signs of damage to show up are usually related to the pancreas, meaning sugar issues. However, everyone is different. But, the elderly and very young are more sensitive. John's Fe was 4600 when he started chelation. It's taken years to get it down. Mostly because he could never get to a full dose of either drug. John took anti-oxidants to help counter the effects of the iron.
Low blood counts can also strain the heart and if your blood volume is low, your body hangs on to more fluid to make up for the missing blood. All this can contribute to edema.
Is she under the care of Internist also? Someone who will look beyond the MDS/iron to see if something else is going on.