I heard the peerlink is a single point of failure in an MLAG pair
What's the best way to hook up four racks each with two ToR CRS326Q?
Each rack of course consists of servers with LAG connection to their ToR switches.

+1 for stacking

https://wiki.mikrotik.com/wiki/SwOS/CRS3xx#RSTP

short: 40G - 1; 10G - 2; 1G - 4; 100M - 10; 10M - 100
long: 40G - 500; 10G - 2000; 1G - 20000; 100M - 200000; 10M - 2000000

What is the port cost for a 2 x 10G LAG group?