If there are four blue strings does this mean that one is the shamesh and one is not, or are they both shamoshim. If they are both shamoshim, when do you switch off?
Only one is a shamash, though if you are in a pinch for length, the mishna berura says that switching off shamashim is not meakeiv. He brings proof from tying tekhelet which requires one to switch between shamashim (albeit, between white and blue). Like I said, ideally you don’t want to switch, but if there is a need, it appears there is a leniency to do so.
– Mois Navon