welcome!
First off, thanks for putting your code inside tags!
Second, I looked at your code and it appears you need to learn a bit more C programming since the code won't compile, as-is. A couple of hints to get you going...
You can't have multiple definitions of the same variable 'hallsensor'. A beginner might use something like hallsensor1, hallsensor2, etc. or a better way would be to use an array like hallsensors[4] (if you are comfortable with that).
All your 'cylinder' variables need to have a type and you can't have spaces in the name so these should all be:
int cylinder1 = 4;
int cylinder2 = 5;
etc.
or again, use an array
int cylinders[8] = [ 4,5,6,7,8,9,10,11 ];
Probably the biggest issue with your code is that you can't just duplicate your interrupt code for all 4 hall sensors. The arduino Mega only hase certain pins that allow external interrups. Check out this link:
Good luck!