AIUI Wightlink no longer has a 'spare' older catamaran as back-up to Wight Ryder I & II, so if one of those is out of action for maintenance or repairs, they can only offer an hourly service at best. In that situation the hovercraft may pick up extra business. Hovertravel seemed to have around 3 or 4 available last time I was at Ryde.
Both companies have two machines.
Hovertravel had two AP1-88s which have been replaced by the new Griffon 1200D machines. The 1200Ds took almost two years to debug so they had to keep the old craft until earlier this year. That will be why you saw four machines together.
Wightlink have the two catamarans and occasionally hire Wight Scene from the Blue Funnel line, particularly when one of the Wight Ryders is away for maintenance.