If you do go with the Number 4, you can do yourself a real favour by saving money.
What you do is load DOWN just a little. I'm using 37 to 38 grains of IMR-4895 in the .303; that's 184 to 189 shots per pound of powder, so it's cheaper.
With a Sierra 180 flatbase bullet, this will give you about 2250 ft/sec at the muzzle, which just happens to be the most accurate velocity for the .303 cartridge with this weight of bullet. Seat them to the overall length of a military Ball round; this will crowd the rifling at the leade, just a bit, and give you better accuracy. How accurate? How does one inch sound at 100 yards, if you can hold it? Sounds okay to me. That's with a decent barrel, mind you, and most Number 4s have pretty decent barrels, even a LOT of the Bubba models.
Nice thing is that this also prolongs the life of your brass, so all you have to do is keep it trimmed to length. That's not much of a problem because likely they will shorten on the first firing, anyway. When you reload, be sure you lube the INSIDES of the case-necks before you neck-size or full-length size your brass (neck-size only if you have just the one rifle: better accuracy AND your brass lasts longer). This will prevent 'drag' from the expander button. I find that MANY .303 dies actually lengthen the brass MORE than does actual firing the stuff; you avoid this with the inside-neck lubrication to a great extent.
BTW, you can change butts on the Number 4 to fit the rifle to your own shooting preferences. They were marked B for Bantam, S for Short, either N or no mark for Normal, L for Long, XL for Extra-long and there even was a Canadian butt marked XXL for Extra-Extra-Long, which fits nicely to your 6-foot-9-inch guys or to your average Orangutan. Me, by rights I should use a Normal, but I prefer shooting with a Short; just seems I can hold it better, that's all. You fit your rifle to your own shooting, and that's something that you can ONLY do with a Lee-Enfield, either the Number 4 or the older SMLE.
Most important point: be sure to have fun!