After reading what you say about system 4 and system 6, it seemed as if it was just two prepackaged sets, but looking around it exists many different sets of each?
What is the difference of system4 and 6?, size of bits, and handle?
One set in Drive-Loc System 4 (can get singles, but it's more costly). Multiple sets in Drive-Loc System VI, as well as blades and handles that don't come in sets at all (the big master set comes closest).
Aha, 4 is what they also call micro bits?, and 6, is that somewhat smaller than the normal 1/4" bits?
Yes, micro bit is synonymous with what they term as System 4 in terms of shaft diameter.
Biggest difference with the Drive-Loc (vs. standard System 4), is both the longer length and dimples on the flat sides of the hex (locking detents). Physical shaft diameter (point to point on the hex) of the blades is the same respectively. No equivalent non-dimpled version in System 6.
Drive-Loc 6 will not work with standard 1/4" bits. They actually fit inside the round part of the hole/sleeve, but do not engage the hex pattern (just spin in the handle). Pressing the dimple release button/collar doesn't solve this (don't have any true 6.0mm shafts to try other than the Drive-Loc). I expect most 3rd party bits are actually 1/4" (0.250"/6.3mm) anyway, but pay attention to below, as there may be a potential issue that does exist with System 4 (read on).
System 4 OTOH, standard 1" long 4mm shaft bits I have will (Kronus brand). Meaning they engage the hex, and will turn a screw. More importantly however, if you push the dimple release collar and push on the bit (overcome the springs on the detent balls),
you can potentially lose it down the handle. And if there's not enough poking out to get hold of it with pliers or something, you'd have to drill the handle to retrieve the bit (going from the opposite end should still allow the handle to function, though cosmetically unappealing).
As it happens, Wiha sells adapter blades, such as one for 1/4" bits for System VI. There's at least a 4mm standard bit adapter blade that I recall seeing in the EU catalog (not sure if there are any others). Not absolutely necessary with System 4, but it would be safer (no way to lose the bit down the handle if you're not thinking/distracted).
There's even a 1/4' drive socket blade for System VI (discontinued selling it in North America, but still available in the EU catalog last I saw). Nice little cost saver IMHO, as it can eliminate the need for most nut drivers if you have the appropriate 1/4" drive sockets (I find mine very useful).