Only having the 4 quadrants is kind of the point. It is simple.
Effectively it allows tasks to be put into 4 high level categories, each having a specific type of action (i.e. Do, Defer/Decide, Delegate, Dump/Delete). It is an initial, high level decision filter on what to do, and just as importantly, what not to do.
Some people leave it at that. Others then prioritize the members of the quadrants (e.g. Do1, Do2, Do3... - although I don't see the need to prioritize the Dump/Delete quadrant..).
Some systems call the quadrants by other names, or use A tasks, B tasks, etc... - hence priority A1, A2, A3,
Note there are variants of the Eisenhower matrix / 4Ds of time managment, and I have even seen one with 6Ds - 6 quadrants (sextants?).