I'll go through the numbers so bear with me as I tend to be long winded
XB1 sales ratio is historically about 1817/3000 -> ~60% US: ~40% ROTW based on NPD numbers for
November and
December compared to
3M sold number at end of Dec. WW
Based on that we can estimate WW sales from NPD totals
Which as of the end of February, XB1 has sold approximately 2218K in the US
Thus WW total as of beginning of March is likely in the ballpark of 3.7M although that's assuming the same sales ratio in February which spikes up the install base due to its great performance in the US.
It is very possible XB1 has sold 4 million WW already. If so MS would certainly know about it so they are either saving the announcement for some reason or they do not wish to announce numbers as you suggest until their earnings reports