You may have to "jiggle it".
Best practice: between each step below, shut down, unplug the power adapter, remove the battery, and press the power button for 15 seconds or so, then make the next change.
First, remove the new module, restart the computer, make sure all 4GB is recognized. It so, shut down.
Next, use compressed air or some other method to clean out that second RAM slot. Dust, lint, or a strand of hair can make just enough of a gap to affect the contact on all the pins. Then install the new module again and test. Pay attention to how it clicks into the slot, be sure it is firmly seated (without breaking anything). Test.
If that does not work then switch the modules. Put the new module in the old module's slot and test it alone. Does it report all 4GB?
If it does then install the old module in the second slot and test.
Whether any of this works or not, run Memtest86+ - Advanced Memory Diagnostic Tool
for at least 3 passes to see if it reports any errors. Set it up before you go to bed and let it run overnight.