I’m surprised it would take 5 minutes to acquire 64 directions with MB4. We can already do that many volumes in 10 minutes without MB, I’d expect this to take less than 3 minutes. What resolution / number of slices are you targeting?
In any case, you’ll probably be able to do 3 shells in 10 minutes with that setup. That will allow you to get >128 volumes, which you can split up in a relatively sensible ratio in the region of 10, 18, 32, 68 volumes for b=0, 700, 1200, 2500 s/mm² (or thereabouts, depending on what b-values you eventually decide to settle on).
One thing I would recommend is to spread these b-values around in the time domain (and orientation domain too if you can) – i.e. don’t acquire all your b=0 together, followed by all your b=700 volumes, etc. This brings benefits in terms of making motion correction easier, makes the sequence more resilient to interruptions, lowers gradient heating, and often allows the scanner to run the sequence faster (depending on the exact implementation) – see this paper for a discussion of these issues.
As I’d stated in that thread, I don’t think the matter is settled as to how many shells are optimal… 2 shells (+b=0) allows you to do multi-tissue CSD with 3 tissue types, but that doesn’t mean that future applications won’t benefit from additional shells (e.g. Fig 10 in this paper). I don’t think this is a question we can honestly answer without speculating…
Same with the issue of which b-values are optimal. This is something we’re looking into, but at this stage, I wouldn’t want to give recommendations, other than just to say that the b-values I suggested above would probably be what I’d go for if I was in your shoes (for a 3-shell setup). For 2-shell setup, I’d probably opt for 10, 30, 70 directions at b=0, 1000, 2500-3000 s/mm² or thereabouts – again based purely on gut feeling rather than any formal definition of optimality…
Finally, on the question of how many directions for a given b-value, the only recommendation I can provide is to ensure that have at least enough to capture (the bulk of) the information content in the signal at that b-value – see this paper for details (it’s a single-shell analysis, but the results do apply to this particular problem).