Is it better to have 4GB RAM in dual channel or 6GB as single channel

"Quad Channel" is just a marketing ploy. It's 4 sticks of matched memory, and is the same as buying 2 dual channel kits. If your motherboard supports dual channel, and it can handle the amount and speed of the memory you purchased, you should be fine. We'd need to know your motherboard model to find specifics on what it supports though.

Edit from an anonymous user:

That was an old response. As with all technology, dual channel has advanced to triple channel and...Quad channel.

DIMM slots of the same colour indicate different channels. In the figure, DIMM1 and DIMM3 are on channel A, whereas DIMM2 and DIMM4 are on channel B. In order to make use of dual-channel operation, both channels should have a RAM module with matching capacity, hence the manual suggests installing two matching-capacity modules into DIMM1 and DIMM2. (DIMM1 and DIMM2, as well as DIMM3 and DIMM4, seem to be paired, so you cannot install matching size modules into DIMM1 and DIMM4, for instance, despite them technically being on different channels as well.) Extra, unpaired RAM modules will run in single-channel mode, no matter their capacity. In the example, that means 4 GiB dual channel RAM from the 2+2 configuration plus 4 GiB of single channel RAM from the single 4 GiB RAM module.

In other words, the 4 GiB size of the other module is just for illustration that the sizes need not match in that case. That 2 GiB + 2 GiB = 4 GiB is a coincidence. The following figure also shows a 2+2 & 4+4 configuration in dual channel, illustrating that the paired slots will continue to run in dual channel mode as long as the capacities across channels match (thus it's 6 GiB per channel in dual-channel configuration).