You should still use an incubator... ball pythons normally maternally incubate their eggs so you'll want them to be at a very steady temperature.

As for breeding a male pastel to 4 normal females, it would all depend on the females in question. Females have been bred at 18 months, but that is not the norm. If they're big enough by then, you can always give them a try though, but plan for them to not breed until 30 months. Also, not every female will take for you, so it is good that you have more than one. I would hope for maybe 2/4 to take? Average clutch size for ball pythons are 6, but since your females will be young, plan for 4 perhaps? So, within 2 years, a reasonable estimate would be 8 offspring with 4 pastels. Just because your odds are 50% though, keep in mind you could get anywhere between all pastels or all normals, depending on the luck of the draw. If getting back your money quickly is an issue, I would hold off on buying the pastel male until all your females are breeding size, and then purchasing an adult proven breeder pastel male. Even then, expect it to take a year to get the male in the same cycle as your females.

Good luck!!! I'm starting off my first season with pastels, so I'll post about stuff I learn along the way.