A good reference for plate blocks is Brookman.. excerpts from cataloghttp://www.brookmanstamps.com/pricelists.html
They have prices for sheets and plate blocks. Now, their pricing is high (meaning you should find it cheaper elsewhere) but is useful for relative prices.
The number is usually 4 unless some special reason (like a second plate number). Most collect by the format defined Scott (4,6,etc).
Example excerpt from Brookman's web site for educational purposes only