ezo
Get off my lawn!
Well, if you really enjoy rolling, the best method I have found for creating a normal distribution is 15d2-12.Nah, PCs are special. This was more an exercise in seeing what a more normal distribution could be for, and how to switch rolling to d4s
For PCs, roll 15d2-10 (AnyDice). This shifts the PCs to an average modifier of exactly +1, with a range of -3 to +5.
As you can see, for PCs you have (basically) the follow chances for the listed ability modifiers.
Edit: of course, you can still roll 18 or higher as a PC, but the chance of a +4 modifier is about 1 in 270, and +5 is less than 1 in 32000.