Thanks for the question :)

My thoughts:

each VM needs to run it's own system and background processes needed for maintenance and connection. If you have 4 VMs then you're running 4 systems, each of them taking same amout of CPU and Ram away from the computation, rest is available for boinc.
Now if you have 1 VM with 8 cores, it runs the system only once, and all the remainig resources go towards BOINC (which is brilliantly optimized) that can run even on all cores of 50+ Core CPU.

The difference might be in pricing. For example running 4x 2cpu for one month might come out cheaper than 1x8cpu (but that wouldn't be obvious, because of more necessary electricity consumption).

There's one way to find out for sure, a little experiment to do.
Make a new google account, register new Pool account.
Sign up for google cloud. Create 3 instances - 1 with 8 cores (8CPU), and 2 with 2 cores(2CPUa & 2CPUb).
Install boinc and connect each of them separately to the same account.
Sign them all up for the same project.
Run them all for a week.
Check your hosts panel for details.

If
8CPU-MAG > 2* (2CPUa+2CPUb)
then
It's more efficient to run 1x 8 core, than 4x 2 core
else
4x2core is more efficient