My computer is four core.
But only one core is used when tested in the ea test. It takes a lot of time to test or optimize.
Is there a way to use all four cores, or accelerate the speed of testing or optimization?
Install the terminal 4 times and run them parallel.
You can even start one terminal 4 times but then you might get in trouble with the files...
A single tester pass can not be performed in parallel (neither in MT4 nor in MT5). If a single pass is lengthy, having multiple cores does not help. But you probably can improve (speed up) your code - try to run profiler and find and eliminate "narrow" places.
Optimization is parallelized in MT5. There exist many MQL-wrappers which allows you to run MT4 code in MT5 without lot of changes (in some cases you only need to insert a single include directive).