4 hours total time is one hell of a time to build up Nitrogen in the body.
Ask them how long they plan to take for decompression stops, this should give you an idea about their saturation. The longer the more saturated.
I teach diving for fun, BS-AC, and we recommend no one flies or drives up high hills until at least a tissue code 'B'. That is at least 4 hours from your last dive (surfacing a 'G' with a small deco stop).
When I was in Saudi there were many cases where divers left Jeddah & drove into the mountains, 2-5,000ft, suffered from bad decompression illness and had to be rushed to the hyperbaric chamber.
I fully agree with crab, the lower the better.
Sycamore has a point, oxygen would help draw the Nitrogen out but if they are greatly saturated they could still suffer.
Once you have suffered from Decompression Illness, you are more susceptible to it, I would check to see if any have suffered before.
As you pointed out, they have said they are 'professionals' so tell them your planned height and get them to sign a waiver against any costs or damages resulting from decompression illness.
On the positive side, seeing as you said they were only going to 20m maximum, they might be diving on Nitrox, higher percentages of oxygen, so less Nitrogen to be absorbed, this still puts Nitrogen in the body though.
Good Luck
FB