Disable Surface Pro 4 Type Cover Fn key toggle behavior

I don't believe that exactly what you're asking for is currently possible, no. I found some discussion and suggestions in this thread (along with many people who did not understand the question!) which may help you solve your underlying problem in other ways though.

One suggestion there was to use AutoHotKey to map the F keys permanently to their desired functions - F1 for Help, F5 for Refresh etc. I haven't done this myself, but have used AHK before and recall it being fairly straightforward.

However, probably the easiest solution is to either:

• Buy an earlier Type Cover (from a Surface Pro 3 for example), where the Fn key required you to also press Caps Lock to toggle between the two behaviours, making an accidental toggling almost impossible (unless you play your keyboard like a grand piano!) or at least much less likely.
• Alternatively, you could get an inexpensive USB/Bluetooth keyboard which doesn't have the Fn toggle functionality at all and use that instead of the Type Cover.

At least on Surface Pro 3 if you press Fn+Caps you will toggle the default state of the keys between hotkeys and the F1 - F12 keys.