A 2 stroke engine is said to be 4 stroking when the mixture is richened enough that it begins to fire only every second cycle. It literally doesn't fire on one downstroke but fires on the next somewhat like a true 4 stroke engine. The exhaust note changes from the usual high pitched scream to a growl. With careful adjusment of the needle most engines can sit right on the transition point and will continuously switch between the two modes.

This transition point is where the running in mixture should be based. If an engine has rings then the transition point is the leanest it should be set, in other words it must be 4 stroking all the time. For an ABx (which yours is almost certain to be) then the correct setting is just barely leaner than the transition point so it's always 2 stroking although I like to hear the odd misfire. It's adviseable to use a small prop for running in to keep the revs up when running rich and to take some of the load off the con rod (which needs to be run in as well as the piston/liner). A 9x6 would be a good size for a 40 engine.

Always use a fuel that's got at least 20% oil and half of that should be castor for running in. Nitro content doesn't matter because it has little affect on the running temp of the engine. There are a number of ways used to finally run them in (especially ABx) but in general aim for about 40 minutes total while very slowly leaning it out but NEVER going to peak revs.

All of this is basically the way both Jett and Rossi say to run in their engines and I figure they know what they're talking about