"Rules or conventions" as in "rules or conventions of OSM"?

You can add the address, plus a note under the `fixme=*` key to say something like "`addr:housenumber=4020` should be confirmed on site," just to flag to anyone who happens by this data to know it's *technically* just speculative.

That said, the conventions of street addressing in Calgary make it overwhelmingly likely that this is 4020:

1) it's on the "4000 block" of Stanley Road because it's between 39th and 40th Avenues, 
2) it's on the east side of the road, therefore it's an even number,
3) it's located equidistant between known addresses of 4016 and 4024. 

To me It's an extremely reasonable guess.