Shoved the GTX 1650 into the 9020 and ran the 3dmark Time Spy benchmark on it both in the 4x slot (shoved inside the case) and the 16x slot (using a riser cable).
The first run in the 4x slot with no settings changed gave me a Time Spy score of 3098, then with the 16x slot 3259. I'll have to try some actual games at some point and see how playable they are and whether the 16x lanes actually make a noticeable difference, but the results indicate only a minor performance loss from the 4x slot with a relatively weak GPU.