Memory is less than the amount of installed memory in Windows 8.1 / 10!
The indication of the memory can be quite confusing in Windows 8.1 / 10, usually memory is always less than the really installed memory!
Now don't panic, if you use a 32-bit Windows 8.1 so only 3.5 GB used.
A 32-bit version of Windows 8.1 report that only 3.5 GB of usable memory available on a computer with installed 4 Giga Byte memory. This is a normal worth under Windows 32-bit usually varies from system to system between 3.2 and 3.51 gigabytes depending on the computer hardware!
But also for Windows x 64 (64-bit), it may happen that the available memory is different from the actually installed RAM. Especially if you have for example on-board graphics card with 256 MB, Windows must assign this memory within the first 4 GB of address space.
There are also cases where the memory chip is actually defective! Or, there are several additional situations that could cause the usable RAM to be less than expected.