Pizza for 4 days in a box in fridge good?

Not yet. Leave it for at least a week.

As long it is not soggy, microwave it on the pizza button and see if it comes out good eat it.

I just ate a four day old slice about two hours ago. I'm OK.

If it has no meat on it it won't kill you but will taste old...............because it is.  I would not eat pizza left over more than one day.  The germs on it when fresh have been multiplying for four days.......................do you really want to eat it?

How To Reheat Pizza In The Microwave Without It Getting Soggy

• Put the pizza onto a plate and then put it in the microwave.
• Next, grab a microwaveable cup and fill it with water.
• Add that to the microwave along with the pizza.
• Switch your microwave to high power and cook the contents for around 30-45 seconds.

https://vivaflavor.com/how-to-microwave-pizza/