sbws: use at least 4 measurements that are at least 6 hours apart

Split off legacy/trac#27346 (moved):

use at least 4 measurements that are at least 6 hours apart, because:

• there is a daily cycle
• each day contains 2 similar points in the cycle (it is an up and down cycle)
• if all 4 measurements happen within a few hours, they will still be biased towards that point in the cycle