We rented from Budget at the CUR airport. As there were 6 of us - a small Jeep and a 2dr pickup truck for tanks. IIRC the Jeep had some sort of plastic liner in back - may have been standard equipment. Nice thing was it had a cargo cover we used when we left equipment back there while diving. We had the truck but I don't see any reason 4 tanks couldn't have gone in back also.
If there was an exclusion for diving, my buddy never mentioned it. I know we did not do any cleanup b4 dropping the vehicles at the airport the morning we left.
Unlike Bonaire - at the vast majority of dive sites on Curacao you will find a dive operation that rents tanks for a nominal fee so you would not have to haul any. Playa Jeremi being an exception - there's nothing there. It's on the north end of Lagun though and Discover Dive about 10mins. south at Playa Lagun. They have a drive up tank loading dock.
At some like Playa Kalki/Alice in Wonderland you want to use their tanks as they're staged 20' from the dive dock. To get down to it is a flight of rough stone stairs - they had a tank trolley next to them but it looked like it gave out years ago.
If you dive the old Sunset Waters site (plane wreck) the resort closed years ago so you'll need bring everything. I think only one of the Gnip sites has a shop although there's 2-3 dive sites along the same road.
Also the dive school at the Tugboat seems to go out on a regular basis. We were able to get tanks from them - I've heard others in later years were not.
Look at the dive listings at
www.curacao.com - a lot of the shop names give away their locations. Or there's Relaxed Guided Dives on the main road heading west - they have an on-site fill operation and rentals.
We did many of the known western shore dives over 10 days of diving. I can't think of any that were very far off the main road or an improved dirt road used by a lot of people daily. At several sites there were small low cars parked while locals used the beach or dove - like Toyota Corolla's or equivalent.