Hi Deezlink.
It's very simple, all you need to do is open up the scenario folder, then copy the scenario you want to use.
Then open a scenario with the DGIV in it, highlight and copy the text which specifies the DG-IV.
Open up the copied scenario, paste it in between the BEGIN_SHIPS and END_SHIPS lines.
Copy the LOCATION and HEADING lines from the existing standard DG, then paste them over your LOCATION and HEADING lines for the DG-IV.
Then cut out the standard DG-IV. Make sure there are no line gaps.
Save your new scenario. It's ready to go.
There's another way too, a bit trickier:
Enter the scenario with the standard DG-IV
Press F1 and choose Scenario Editor
Press New
Choose a Delta Glider IV from the list of ships
Choose Edit Ship
Enter Edit, then Location
At bottom right there's a window "Copy State From"
Choose the DG to copy state
Then press F3 to choose the new ship
Then go back to Scenario Editor
Delete the Standard DG
Save the scenario