Why not use 3x4 instant prints in a Polaroid 405 holder? It will be much cheaper this way.
Of course, since you are talking about buying a holder in any case, you could actually buy the proper Fuji holder for their 4x5 instant prints.
However, one thing I would not do is to try to put Fuji instant prints into a 4x5 Polaroid holder. The only thing these holders might be good for now is for using Fuji Quickload film...and even this use is not ideal.