Your instinct about the low-mileage car is the right one, and it is the thing most buyers get backwards.
Engines and gearboxes wear with use. Almost everything else degrades with time and with sitting. Rubber perishes, seals dry out and leak, brake discs corrode, fluids degrade regardless of miles, batteries die, tyres go hard and crack. A car that has done four thousand miles a year has spent nine years doing all of that with none of the compensating benefit of being run regularly.
So the nine year old car is not lightly used, it is old, and it has probably had the maintenance schedule of a car that never seemed to need anything. That is where the cost is hiding.
Meanwhile ninety thousand miles on a four year old car is high but entirely ordinary for the engine, particularly if they were motorway miles.
What actually decides it is the service history, not either number. A stamped, on-schedule history with cambelt and fluids done at the right intervals tells you more than the odometer. And on the low-mileage car, look for time-based servicing rather than mileage-based - many people with low mileage skip services because they have not hit the miles, which is precisely the wrong reading of the schedule.