Does entry views tracking allow you for 4 instances all together or 4 instances per channel?
I know we talked about this on Twitter yesterday, but looking at the EE docs again it says:
"Up to four different instances of the view counter can be used (each on a different page), enabling you to track different aspects of an entry independently"
Which isn't immediately clear (at least in a pre-coffee state). So I did a quick test on my local site, implemented a view_count variable, and looked at the query to see where the data was getting stored. Turns out the view counts are stored in the exp_channel_titles table where there are 4 columns for tracking views.
So my answer is neither four instances total, or four instances per channel, but rather 4 instances per entry.
Be careful on using entry tracking on extremely busy web sites. exp_channel_titles is a generally heavy read table, add a lot of entry tracking on a busy site and you turn that into a heavy read/write table. Using InnoDB or similar replacement engines will help, but stock MyISAM table type and heavy read/write contention on that table will cause performance issues.