On the Dell Support website, enter your Tag number and select "System Configuration", then select either "Original Configuration" and expand the "Components" list or you can select the "Current Configuration" tab and have Dell scan your system. I've never used the "scan" option, so I'm not sure what information it provides. Under "Original Configuration" components list, you can scroll through it looking for information on memory (you may need to expand the line item to see it). Look for a line item with "DDR" in it.
If your Dell laptop has a memory access door, you can physically check to see if you have an open slot or not. On some laptops both memory slots are together, but on others it will have one module on the bottom and the other located beneath the keyboard.
If you have 2x2GB, then yes you should be Ok with removing one 2GB module and installing a 4GB module.