A 4" round serves 6.28 industry standard portions (1x2x4" -- 8 cu. in.). If you are making it 8" tall, it would serve double if you use a board in between to cut as 2 separate 4" high cakes.
Dollar tree! Red plastic bottom half, clear top, a snap on each of 4 sides with a handle on top, indents in base to hold 9... they're more than great for only $1.00 and reusable (rather than Hefty foil containers).
More info about full sheets (I wrote this in another thread):
A commercial Bun pan is 18" x 26" (outside measurement), and because they are tapered for nesting or making them stackable, the inside measurement is 16.5" x 24.5".
A commercial full sheet is 16" x 24". They are baked in 16" x 24" bakeable cardboard trays that fit into the Bun pans (flat surface portion) which are used during baking for support and handling purposes.
A true commercial full sheet (16" x...