It would certainly be easier to get the four speed shifter than trying to modify the five speed shifter to work! Also, the shift cables from a FWD car won't be long enough to reach the Fiero transmission. The 4 speed shifter from a Sunbird or Citation might work... although I'd be willing to bet the shift pattern is reversed on those. Surely someone must be selling a Fiero shifter in the Mall. You can get the proper new cables from the Fiero Store for a decent price.
You might be able to make any of them work with some mods, but as they are, they won't work. FWD cables are usually shorter. The Fiero cables all have a different retainer at the transmission, that being one cause for not interchanging. Some have converted the 4 and 5 speed Fiero shifters to work on the other, but it probably would require some work. Isuzu and Getrag shifters are interchangeable, but the 4-speed is designed for reverse to be left and up. Just opposite of the 5 speeds. I have a good 4-speed that came off one of my 85's when I converted them to Isuzus.