There is no PDF 'Mazda' Service Bulletin on the '3/4 adjustment procedure', it is not even in the factory P66M WSM.!!..

Page 05-11-37 of Mazda P66M WSM says..
If there is an abnormality in the 3rd/4th
shift stroke after assembling, loosen the
locknut and readjust.

But it does not explain in any detail the 'readjust procedure' to get rid of the abnormality, Jason's explanation does.
That is to measure and then equalize the distance between 3 and 4 shift lever gate, then once satisfied are equal, lock the locking nut.

I might make a PDF one day...

Here is the Miata link showing the procedure.
NC 6-Speed Transmission Adjustment Procedure DIY - MX-5 Miata Forum

 

Sorry to revive an old thread, but this just happened to me. 2010 RX-8 with only 23,000 miles. While driving along steadily in 4th gear it would sometimes pop out into neutral. I have 6 months left on the 5 year powertrain warranty, so I took it to the dealer. I showed them the Miata procedure, but they ended up tearing the transmission down and said it needed a "3-4 gear set and synchronizers and counter shaft."

Luckily it was all under warranty! They said it was not a common problem (although the board seems to indicate that it may be). Surprised it happened with so few miles, but I have a short commute with lots of traffic (shifting) so maybe that's what did it...

 

If it's under warranty, I'd have much less as far as complaints, but it's unfortunate that, if it's an easy fix, they would replace a whole bunch of stuff unnecessarily... but if that's the way for them to completely eliminate a problem they can't figure out AND it's under warranty... so be it...