I recently had to do this myself because I was in the same boat.
This is what I did (working from memory here)
1. Installed GenX to the default location which should be Program Files/DAZ 3D/DazStudio4
2. Launch D/S.
3. Go to HELP>About Installed Plugins.
4. Scroll down until you see Genesis Generation X. Beside it, there should be a long bar where you have to enter a serial code.
5. Go to your “My Account” on the DAZ website. Locate “My Serial Numbers”. Go to where you should have a Serial Number for GenX. Copy it with your mouse.
6. Go back to Daz Studio and enter your serial number in the empty box and click on the symbol next to it.
7. If it asks for verification at this point or when you try to use GenX, enter your Rendro username/E-mail and the Order number that you ordered Gen4.
8. Your Rendro order number for Gen4 can easily be found on Rendro. If you don’t know how, sign in and go to “Members>My Account” then scroll down until you see “Search Perchases”. Enter Gen4 and click Go/Search. You should find it very easily. The number beside it is your order number. Enter that in the required field in D/S. Click OK.
9. Now within D/S go to Windows>Panes>GenX
Should work now.