I am still thinking about if I should use SPS or wood dowels.
But if I use wood dowels, what size of dowels do I need to use to support for 4 tiered square cake, 12" bottom, 10" 8" and 6"?

You're kidding right? SPS is a lot more secure and easier to use.

Well it depends on how tall each tier is going to be. "I" prefer to use the Wilton ones rather than the ones you see in the craft/hardware area because I find it cleaner.

You can also try out bubble tea straws.

I use the Wilton ones and it works out well, even with 4 feet tall cakes.