Plywoood's construction of alternating layers, each layer has its grain pattern rotated 90degrees from the prior layer. This effectively eliminates expansion/contraction, thus you can put each piece of plywood as tight as you want against another piece of plywood (talking edge against edge).
One thing to consider is where the seam created by butting two pieces of plywood together is created. Is the seam supported sufficiently for your application? For an extreme example, if your 1x8 foot piece butted against your 4x8 piece creates an unsupported seam and you try to stand in the seam area, you foot would probably go through the seam.
