You can measure availability A=(Total time expected service to be available-Total outage time)/Total time expected service to be available.
The above formula works good to measure individual component availability, but when it comes to measure service availability dependent on 4 components then you need to multiply individual component availability to get overall service availability.
Ex. Service A availability=(Comp A %* Comp B % * Copm C % * Comp D %)
=(99.9% * 99.9% * 99.1% * 99.8%)
I hope this helps you