I had 4000 gallons pumped out of my cesspool about 2 months ago and when I flush my toilet when the washer is on or we recently had showered the toilet fills up and flushes slow can someone tell me what this could be?

Many things, perhaps just blockage in the line under the house or toward the pool.

Have you checked to see if the pool is filling back up or over flowing ?

Yes we checked the pool and it is full again

I have a question, Is this a cesspool or a septic tank with a drainfield? Let me know, Tom

I believe it is a cesspool, my home is over 95 years old, we have lived here 9 years. It is brick inside

My code considers cesspool a temporary band aid until a regular sep[tic system can be installed. Unless you want to keep pumping the cesspool every time it fills up you must either install a septic tank and drain field or add a drainfield to your cesspool. Good luck. Tom

What could be causing it if I have a leach field? I do have that I believe I just forgot to put that with my prior answer.

If you haved a leach field and you still have to pump the pit then the leach field is saturated and must be moved as it can no longer disperse the liquid the cesspool sends it.
Have the leach field checked and if it must be moved get three estimates, Good luck, Tom