PAPI is a point-source. If you maintain a constant PAPI picture, you must be flying a constant descent angle. The aim point will be exactly the same regardless of whether you're flying 4 reds or 4 whites, or two reds and whites: where the PAPI globes "cross" the runway. Only the descent angle changes. For multi installations, if you hold one set constant, the others must be changing. There is no way to deliberately change the threshold crossing height unless you have a changing PAPI picture as you get closer to the runway. Not so with the T Vasi. A far better visual landing aid.
Intruder,
Varying eye height may "not be an issue" with PAPI according to ICAO, but with almost 10 years flying the 747 I can tell you that 3 white and 1 red is the PAPI picture we see on a normal approach.
What are you basing your approach on? The ILS glideslope?