8x4 is better.

Good time to prepare may be in the fall when it's comfortable. Avoid
working when the soil is wet as mud or dry as cement.
Use current soil if it's any good. If mostly subsoil, you might and to
put topsoil on top. You can pile the manure on the subsoil and add
topsoil and compost on top. Look up requirements of your crops as some
plants don't produce well after too much manure. Unless compost is
free, I would use 10%. Mix well.

Most garden books are pretty much the same. Any one from the library
will do.