If the sqlservr.exe startup accounts have not been given the Lock Pages In Memory right, you can use Perform's Processes\sqlservr Private Bytes and ID counters. The Windows' process ID is identified in SQL Server's errorlog, during startup.

Or whether or not the sqlservr.exe startup accounts have Lock Pages In Memory, you can use SQL Server's Buffer Manager Total Server Memory counter.

However, unless you like playing "whack a mole" with memory consumption, you should be capping max server memory for each of the 4 instances, so that in total they do not exceed the physical memory on the box (after leaving some memory for the OS and other significant memory consumers, such as SSRS, SSIS, etc).