It is normal. Due to PC architectural requirements such as OS, applications and memory shared for onboard graphics, the left available physical memory will be various and less than total physical memory.
Installed Physical Memory : 4GB ( The memory installed physically.)
Total Physical Memory : 4GB ( OS reported and can be used. )
Available Physical Memory : 3.55GB ( This is left available memory out of total physical memory. It will be changed depends on OS, application, onboard graphics and so on.