I'd suggest that you get two XBees. Program them to report RSSI values, and display the values on an LCD. Move one Arduino/XBee/XBee shield/LCD device around, and see how the RSSI values change.
This will tell you whether, or not, in your environment, the RSSI values means anything.
I understand the strength of the XBee's signal is coded only on a 5-steps scale,
I never said that. I said "like a...".
why would a bunch of receivers be needed ?
The worse the resolution, the more measurements you need to take to discern ANY difference.
Couldn't one apply triangulation with only four receivers ?
If all you are interested in is "in the same room", "in the same building", "in the same town", "in the same state" kinds of data, triangulation will work fine.
Then, could Bluetooth or Wi-Fi have a finer scale to evaluate a signal's strength ?
They are all radios, subject to exactly the same limitations. You have a portable radio? Take it outside and walk around the yard. As you move closer to, or farther from, the transmitter, does the signal change noticeably? No, of course not, unless your yard is many orders of magnitude larger than mine. The same problem exists with any device trying to measure signal strength. Strong enough to receive vs. not strong enough to receive is about the only measurement that is accurate.