For my generator? It's a duromax xp4850eh with one of those 240v L14-30 outlets. The inlet is about 75 feet away from where I want the generator to sit outside. Because of voltage drop concerns, I thought maybe I could connect a 8/4 extension cord to the generator first, then connect my 25 feet 10/4 cord to the end of that. I really don't want to deal with a full on 8 gauge 75 foot cord.
Yes, you can run extension cords in series. The wires have to be sufficient size for the ampacity from end to end. The cords and in particular the junction between the cords would have to be rated for the environment, they'd have to be weather proof if they are outdoors.
The voltage drop would be additive, you'd calculate voltage drop from the source voltage on the first cord, then calculate the drop of that adjusted voltage on the second cord.
There is no issue with a 10-4 cord at 30 amps for 75' - that would be less than 2% at 240V and less than 4% at 120V, which should be no problem at all.