Michael, I've been reloading since the early 70's. I started out with "Lee" tools and equipment, sometimes against friends better advice, and never have regretted it. "Lee" makes great tools at extremely inexpensive costs. I see you've already learned the "Lee" dies will work in your "Dillon" press, but you will find some things you'll need are "Dillon" proprietary, excellent equipment (and expensive). But for now, 'dance with the one ya' brung' and you won't be sorry.
Now, for your original question .......... by all means get the 4 die set. They are exactly the same first 3 dies including the carbide sizer as the 3 die set, but they also include as the 4th die a "Lee" factory crimp die. It not only provides a much better crimp, but will do a roll crimp as well as a taper crimp. At the same time it's providing the crimp you need, it will resize the case through a carbide ring to ensure that every cartridge will fit your gun, regardless of whether it was fired first from your gun, or if you picked it up off the ground at the range.
The 3 die set does any/all crimping by use of the bullet seating die. It will do the job, but not nearly as satisfactorily as the factory crimp die. It can and sometimes will actually crush a case while crimping if for instance you're trying to roll crimp a case using a bullet without a cannelure. It will work 95% plus of the time, maybe more, but will occasionally ruin a case.
If you're loading for example 9mm or any cartridge that head spaces on the case mouth, it doesn't take a lot of deformation to the case mouth to run into head spacing problems especially if the bullet does have a cannelure groove on it. Yeh, it can be done but for the difference in cost of the 4 die set over the 3 die set, go with the 4. If you're loading something that head spaces on the rim like a 38spl or .357mag, then a roll crimp, which is what the 3 die set does, is perfectly fine as Snake45 indicates.
And as an extra, most folks, especially competition shooters believe the taper crimp leads to somewhat more consistent and accurate ammo than a roll crimp. That's something only a factory crimp die and/or a collet die can get you, but every shooter has to decide for themselves. I'm basing that last statement on the fact that way back when I was shooting a lot of competition (1980's and 90's), my wife and I together was shooting sometimes as much as 15-18 thousand rounds per year around the SE and without question I found much better accuracy over all when using a taper crimp made with a FCD.
Hope that answers your question, and I'll add one last little tip. Always clean your cases well before using any sizing die whether carbide or not. Dirty cases will eventually scratch the inside of your die and from then on it will scratch up your cases whenever you load, and sometimes will actually cause a case to stick ..... hard!!! You do not need to lubricate any pistol case when using a carbide sizing die, but they must be clean.
Hope this helps.
www.midwayusa.com
