I'd like to know what are the factors that need to be set in order to sustain the Dual Channel mode.

In a mobo with 4 DDR3 slots:

1. Do I have to put pair of chips? Eg: If I put 3 identical chips only, will I have Dual channel or not?
2. If I put 4 Ram chips that aren't from of same ventor/model, will I have the same latency among them (the highest of all)? Also, will I sustain Dual Channel mode?
3. If one Ram has max frequency of 1033 and the other 3 chips are of 1300, will I have 1033Mhz for all chips and Dual Channel mode on?
4. What if I put 2x4Gb and 2x8Gb chips (latency, Dual Channel)? Can I put 4Gb chips in slots 1 and 3 and 8Gb in slots 2 and 4 and still have dual channel mode enabled?

(Some of the questions might sound silly but their answers aren't that clear to me)
(Also, assume that there aren't any bottlenecks because of other parts on the system)