Because when you cut the case down the area of the case wall that is now the "mouth" is much thicker (i.e. the case mouth is now less than .458 in diameter). If you try to seat a .458 slug in it without inside reaming it, you will get a big bulge in the outside of the case and you can bet your bottom dollar that your bullet will be off center to the bore.

If you fire form the shorter 350 Rem Mag, you don't need to trim nearly as much length down and the case neck/mouth is thinner anyway because it stretches from 358 to 458.

4198 is certainly the easiest way to get where you want to go. You could also try SR4759 for loads in the 1,700 fps range.