Why SWR is the ratio of maximum to minimum voltage along the feed line, and it is caused by reflected power from a load that does not match the line's characteristic impedance. A perfect match gives 1:1; any ratio larger than that means some power is being reflected back toward the transmitter. A 4:1 reading corresponds to a load roughly four times (or one fourth) the line impedance, such as 200 ohms or 12.5 ohms on 50 ohm coax, so it signals a significant mismatch. Most solid state transmitters start folding back power above about 2:1.
Watch out SWR is a ratio, not a decibel figure, so it never directly states gain or loss, and the 'good match' choice describes a reading near 1:1, not 4:1.
SWR: 1:1 is perfect, bigger number means bigger mismatch. The colon tells you it's a ratio, not dB.