Not enough memory to run this program Error message in Windows 10

I’m using a desktop with 4GB memory (3.4 GB usable) on Intel Core-i5-2310 CPU, 32-bit Windows 10 Pro. Some times I get the error message like "Not enough memory to run this application" when I start an application. When I see Task manager, the total memory occupied by all the processes comes to 1.2 GB. And it shows around 54% memory. If that is the case the total usable memory should be only around 2.22GB (= 1.2 x 100/54). But the total usable memory is 3.4 GB. Why is Windows not using that available memory of 3.4 GB for running the application?