Accuracy of wireless quality indicator
4 bars = 100% signal
3 bars = 75% signal
2 bars = 50% signal
1 bar = 25% signal

Why is it that you have signals with 4 bars and you can not connect could be other problem than signal strength.

I use the RSSI (received signal strength indication) as indicator, and the Noise as second indicator on signals that show >50%. With those 2 values one can decide if the WiFi is worthwhile.

The most telling number is the Transmit rate, that shows how high is the actual data transfer.



In this example one has 4 bars signal, but it is not useable, due to to very high noise and RSSI.

The most telling number is the Transmit rate, that shows how high is the actual data transfer rate.

If you hold the ALT key while clicking on the WiFi icon you will see the additional information on the currently connected network. Look at the Transmit rate, the higher the better ie >100 (It is the speed of data transfer).

Mac has more detailed Network analysis tools. In the Spotlight type Network Utility to find  it.