In the Memory control panel, you have the opportunity to set the amount of virtual memory you would like iMac to use. Out of the box, virtual memory is set to 33MB. That is, iMac is set to treat 33MB of existing storage space on the internal 4GB hard drive as random access memory, giving you more memory in which to run applications.
The 33MB reported in the Memory control panel does not, however, account for the 4MB of memory (approximately) required for the Mac OS ROM file. That 3.7 MB of memory is reported in the dialog box that appears when you select About this Computer under the Apple menu. That is, 33MB of virtual memory plus 3.7MB for Mac OS ROM equals 36.7MB.