Actually, crunching the numbers on the skills thing, aiming for about a two-thirds success rate means you want the kind of character that's supposed to find the skill check a decent challenge means you want success on an 8 on the die.
With the proposed modifiers for a first-level character, that looks like:
DC 4 - Very Easy, it's almost impossible to get a lower modifier than -4 so pretty much everyone succeeds 65% or better of the time
DC 8 - Easy, a character with no particular aptitude or skill can do it most of the time
DC 12 - Medium, a character with a reasonable level of aptitude and skill can do it
DC 16 - Hard, a characracter needs both aptitude and expertise to reliably do it
Thereafter, it does seem reasonable to carry on using the 4-point increment, rather than the 5-point increment, to judge the difficulty of a task.