If the M4500 uses the same temperature thresholds for fan control like the M6500 does (most likely it does), the fan will kick in at 60C and switch off at 50C.

If you want to change this hold Fn + Shift and type 15324. The scroll LED should blink now. Then hold Fn and press r and a hidden BIOS menu should show up where you can change fan speeds and disable the BIOS fan control. This setting will last as long as the laptop is connected to a power source. If you disable the fan control it will stay in its current state, so you'll have to change the speed manually (watch out for the temperatures!).

After being glad to have a silent laptop for a while you'll realize that always having to look for the temperatures on your own is nerving. If so, try i8kfan [1]. You can use it to set your own temperature-based thresholds. Adding 10C to both thresholds is fine. Even 20C shouldn't cause any long-term problems. It will not overwrite the BIOS settings but use its own thresholds. Therefore disabling the BIOS fan control is still necessary. It may be old but it works fine on the M6500, at least under Linux. So I guess it should also work on the M4500.

[1] i8kfan