Bluetooth 4.0 is an improvement in pairing protocol and power consumption, but not RF interference.
Expect 2.4 GHz interference, especially from WiFi channels 11 to 14, and from microwave ovens, or microwave communications towers.
afaik all wireless mouses use the 2.4GHz channel, and even if you use 5ghz for wi-fi, if you also live in a dense residential building there got to be other 2.4ghz interference not including the microwave.
There is some speculation that the chipset providing both Bluetooth and wireless contributes to the problem and or the antenna arrangement may be suboptimal so by disabling the internal Bluetooth and using a USB wireless mouse you may achieve better results.
If you decide to try this, let us know how it works out.
I agree that wireless congestion and interference from other sources can impact your experience but to what degree is more difficult to asses. I used a wireless spectrum analyzer app and fount 6 access points (mine and 5 others) on the same channel with numerous others on the other channels. That's not accounting for any interference from phones, and other devices. Yet I'm getting very good performance, not max but the wireless speed is higher than my Internet connection speed so it don't really matter.
I once had a set of wireless speakers for my TV that would pickup my neighbors telephone calls when they were using their phone... you could only hear one side of the conversation for some reason though. I got rid of those.