If I read the question as :

• Are my 4 "I/O blocked" threads keep busy my 4 cores till they complete their tasks ?

Answer is no.

This not about Java, this is about OS Scheduling. (so technically answer depends of your OS, but they all do multi tasking since very long)

So in your precise example, your 10 "fast" tasks will well complete while your 4 "long I/O blocked" tasks are still "processing", on a 15 threads pool size executor service. (whatever is the physical number of CPU/core, or the Java version)