The thing that decides more of these than any of the above, and which people inspect last: get underneath both cars.
Corrosion is what actually ends most cars in a wet climate, and it is a function of age, salt exposure and where it lived - not of mileage at all. A nine year old car has had nine winters. Sills, subframes, brake lines, suspension mounts, and the area around the rear arches. Surface rust is normal; flaking, bubbling under paint, or anything soft when prodded is a different matter.
Brake lines in particular are worth a look on any older car, they corrode from the outside and are an expensive and non-negotiable repair.
Things I would do before choosing:
Pay for an independent inspection on the one you prefer. It costs a fraction of one repair and it is the highest-return money in the whole process.
Check both against a history service for write-offs, outstanding finance and mileage discrepancies.
Look at what each will cost to insure and tax before you decide, since these can differ enough to change the maths.
And the honest summary: given the two you describe, I would look hard at the high-mileage four year old with good history first, and treat the nine year old as a maybe that has to prove it has been maintained on time rather than on miles.