how to determine post size to support a shed roof?

I'm trying to figure out what the correct size needs to be for a shed roof support posts. i.e. 4x4 posts every 6 feet or 6x6 posts every 8 feet. The roof is a 4/12 pitch and the dimensions are 30 in length and 12 width. This is a repair of some prior work. right now there is 4x4 posts every 6 feet and I'm just curious if the 4x4 are sufficient support..