Yeah depends on the question.
We need 4 satellites in good geometry to resolve a 3D position, but, with only 4 we can't be sure of the solution's integrity (i.e., to be alerted if one of the satellites is faulty and transmitting bad data).
RAIM:
With 5 satellites, the extra satellite can be used to detect a fault. This is the RAIM Fault Detection system (aka traditional RAIM). RAIM FD can alert us if there's a problem, although it can't tell which of the 5 satellites is faulty.
With 6 or more satellites, a more advanced RAIM function can detect a faulty condition, determine which satellite is at fault, and exclude it from calculations. (FDE: fault detection and exclusion). The assumption here is that there's at most one faulty satellite.
AAIM: some systems can use the barometric altimeter, INS, or other aircraft sensors as a substitute for one satellite in RAIM calculations. I.e., with baro-aiding, we only need 4 satellites to get RAIM FD, and 5 satellites for FDE.
WAAS (SBAS):
WAAS uses a series of ground stations to detect GPS faults and compute various error parameters. The results are then transmitted via additional satellites in geosynchronous orbit. So if WAAS is available, it can be used instead of RAIM to provide integrity alerts.
In most jurisdictions we need either RAIM or WAAS to use GPS as the primary means of navigation. So that can be a minimum of 5 GPS satellites (to get RAIM FD), or 4 GPS satellites + AAIM (also RAIM FD), or 4 GPS satellites + a WAAS satellite.