There is a definate grain to sheet goods. Look closely at the surface, you'll see the grain.
The best way to bend is perpendicular to the grain, so the grain lines cross the bend. In your case, you'd want the grain to run the 3.75" direction, so you can bend the edges against the grain.
The other thing to look at is the radius of the bend. For most aluminum sheet, the inside radius of the bend should at least be equal to the sheet thickness. If the alloy is higher strength and/or heat treated, the radius goes up to double the thickness (or more). Bending along the grain makes these radii even more important. You may find that you have to go to a 1/4" radius to keep from cracking.
Are you bending the sheet with sharp cornered vise jaws? If so, swap in a piece of wood with a slight radius on the corner and see how that works.
1919 13" South Bend Lathe
1942 Bridgeport M-head Mill