Imagine if the Wii was released during this gen - people would be thinking that the Wii would end up outselling the PS4.

Exactly.  Global sales never tell us the whole story. Differing launches region to region and differing regional buying habits can make a big impact, especially when comparing early-life sales. That's why I prefer to break things down to the regional level. But even at the regional level, there are still caveats. What time of year the system is released, when price cuts are issued and how big those cuts are, stock issues. Having strong early-life sales may be a good indicator that a system is performing well, but frequently fails to be a predictor of lifetime sales performance. Systems all have differing sales trajectories throughout their lives. One only needs to look at the 360 vs. the Wii from last generation in the U.S.:

Launches aligned, the Wii managed to attain a massive lead of about 15.5M units, but that lead declined as fast as it grew, and the 360 eventually outsold it.

Granted, that's an extreme case, but at the very least it tells us that global early-life sales are never going to give us good indicators of how any two systems will stack up against each other several years down the road.