25% idle on a four core machine means that the average core was 25% idle. That could mean that three cores were used all the time and one was idle. It could also mean that all four cores were active 75% of the time. It can mean anything in-between.
What command shall I use to see the total CPU usage of all cores?
It's just math. If you have 4 cores and they were 25% idle, the total CPU usage of all cores is
or 3.0 -- the equivalent of three cores at 100% usage.
If you want to see individual cores, the easiest way is to run
top and then hit the
1 key to show all cores. You can then use
H to show threads and
f j <enter> to show which core each thread last ran on (the