iirc Simcraft has some issue with modeling stat weights with the 4pc correctly. I think Kilee had to come up with a specialized cast list to do the testing he did with it and I can't remember at this point if Althor said they did or did not tweak the action list for the 4pc by default. Either way like veiledy0 said crit is not better in general than mastery.
Well the GCD gets used up in the cast time of MB after you cancelaura so you can immediately queue MS to start casting after you start to cast mb; whereas if you just twiddle your thumbs a minute you'll have to wait on the GCD before you cast MS.