I am new owner of M4500. It is quite good machine bug has a few glitches. One of which is strange backlight control. This model is equipped with light sensor that can control backlight automatically. The problem is I cannot turn off the light sensor without loosing the ability to control backlight myself.
turn off light sensor (fn+left) -> fn+up/down does nothing (I think this is the problem)
turn on light sensor (fn+left) -> fn+up/down works but light sensor too (you cannot control backlight precisely because it keeps changing)
I think it is some kind of bios issue. Could someone with M4500 confirm this?
Dell has changed the function keys on some new laptops. Try the F1 to F12 keys without using the FN key. The F4 & F5 keys might have a brightness control as my new 1764 does. Check them out for symbols. The top symbol doesn't require the FN key, but the bottom one does. This fn key/no fn key setting can be reversed in the bios. Refer to your manual to see if this applies to your model.
Edit-- I just checked your model and this doesn't really apply to your computer, but it might help some other new laptop owner.
Thank you but I am sure fn+up/down are the correct keys, they even work when light sensor is active