The symbol in the top bar always shows the 4 bars, when WIFI is activated. Is this normal, or should it (as on iPhone) represent the WIFI strength?
This is normal - it does not represent the signal strength, but only whether it's active or not. (There's also an animation when it's attempting to connect to a WiFi network)
This behavior was changed in the Windows Phone 8.1. Now the WiFi signal on top of screen shows the power strength as in the WiFi settings menu.
It will always be full when connected, regardless of signal strength. Here is a break down of the three Wi-Fi related icons in the status bar (full list of status icons):
Connected to a Wi-Fi network. The icon is animated when your phone is trying to connect to a Wi-Fi network.
Wi-Fi is turned on, but your phone isn't connected to a Wi-Fi network.
Cellular data connection is shared with other devices over Wi-Fi.