WARNING, WARNING, WARNING...
In the interest of NOT "assuming" while someone makes an EXPENSIVE mistake, you do know that doors only have so much width of "real" lumber around their edges, right?
IOW, I would NOT just cut 4" off ONE END of your doors, there's a good chance you'll hit particle board all the way through and have it exposed on the ends. Better to cut half the total off EACH END - if you STILL hit particle board instead of lumber, then it would have happened ANYWAY. If not, you have a good chance of keeping "real" lumber for the outside edges of the door, which would be much better... Steve
P.S. - If you already knew that, then please don't read the above...
Another thing - if using a skil saw or Radial saw to make these cuts, and if there is a side of the door you want to look nicer, put that side DOWN so the blade cuts UP from the bottom - generally, the side the teeth ENTER when making the cut tends to splinter LESS.
If using a table saw, the side that needs to be less splintered should go UP...
Watch out for some of the "cabinet" type blades available for larger saws - some of these are "hollow ground", and use square cut teeth that have no "set" - relief for the blade to keep from binding in the kerf is accomplished by the hollow grind, but this only extends to about 1 or 1-1/2" deep. This type blade should NEVER be used to cut deeper than the hollow grinding extends, or it will bind and burn, taking the temper out of it. These blades are recognizable by looking at the side; there's a distinct circular line about an inch toward the center of the blade from the teeth, and the teeth don't "alternate" from side to side...
I'll eventually be cutting MDF, so I'm assuming the same will apply that that.