Keys are programmed to the vehicle in the factory, yes. So key 4 from another vehicle won't work with your vehicle, no. It could be programmed to work, but any other key(s) already programmed would then stop working.
A BECM is programmed to a lockset... each lockset consists of up to 4 keys, each with their own ID. The fob code programmed to the BECM is the ID for Key1 of a particular lockset. The becm will then accept that ID and the following 3.
Key 1 and 2 do relate to the seat positions on a vehicle with memory seats. You can save and recall the memory with the buttons on the side of the seat. If you superlock the vehicle, then 'lazy unlocking' it (press and hold fob button) will then move the seat to the preset number of the key used (1 or 2). Key 3/4 will just unlock the vehcile and won't move the seat.
You can't program multiple lockset to a vehicle, nor can you reprogram a key to work with another vehicle.
I have a fob tester, that will capture the fob code (usually) and from that, I can work out what key you have for your vehicle, and also what fob code should be stored in the BECM.
'06 4.2 Supercharged Black/Cream "Jess" -- Bought April 2021 (NZ)
'01 4.6 Vogue Black/Lightstone "Snog" -- Bought Jan 2012
'99 4.6 HSE Cobar Blue/Lightstone -- Sold March 2009
'95 4.6 HSE Epsom Green/Tan -- Traded June 2008