whereas task manager is telling me there is 1633MB of Available memory, which would lead me to think that the server is using roughly 16.5GB of RAM.
You should add the number of memory shown in the green bar in Task Manager + the value of "Cached" below to get the amount of memory that Windows consumes. In your case this will be about 16.4 + ~800 = 17.2 GB used inside Windows.
(For some more details of how to read the Task Managers fields and what they actually mean: http://rickardnobel.se/windows-2008-r2-memory-counters-in-task-manager )
I find it hard to believe the Exchange server is only really using 4GB of RAM unless the VMware Active memory stat is really focused in terms of the period it regards memory as Active, by this i mean it does literally looks at what is active a the point in time the stat is read and not the last 5 mintues etc?
The real exact algorithm used to get the "active" memory is not known as far as I know, but it is something like a very average sample of the memory read or written from inside the OS in the last 15 minutes or so. In your server you can see that the "active" is most often around 4 GB, meaning only that during the sample interval approximately 4 GB of RAM was accessed. We have no indication of where this memory was located and no idea if it was the same memory as the 4 GB of "active" one hour ago.
In theory it could mean that your server at one point in time is very activly using memory pages 0-4 GB, and at some other time uses 5-8 GB and then 9-12 GB and so on.
What could be said is that if the amount of active most often is lower than the total amount of RAM then it is likely that the application has enough of RAM. If the VM would have, say 4 GB of vRAM in total, and you would see the active being 4 GB, then it could indicate that it very often must use all of its available RAM. In that situation it would also make sense to check out any internal Windows paging.
Also in your case, just to see that both Windows and VMware counters actually do show the same: we saw earlier that Windows inside reports using around 17.2 GB of RAM (from formula green bar value + cached). If you look at the vSphere performance counter called "Consumed" this shows also 17,2 GB.
Consumed is the amount of physical RAM that is used by a VM, however sometimes confused with "active", leading to Windows and VMware administrators not really understand each others.