I've got a tach (for experimenting with fuel mixtures, prop sizes etc) and I've never even thought of using it to tune my 4 stroke 1.20 ASP, nor do I have any idea why anyone would need or want to. If you can tune a 2 stroke by ear you can do a 4 stroke too, and trying to tune an engine to get the last n'th degree of revs out of it is asking for a dead engine. Peak the revs out by ear (surely you can tell when an engine's getting faster or slower by the sound?) then back it off a few clicks, make sure it's running smooth, then leave it alone and you'll have a reliable, strong engine that'll run for years. Remember, if it aint broke, don't fix it!

As for valve lash the only time I've had to adjust mine was when I pulled the engine down to check for a bent crank after a somewhat vertical "landing", however from having worked on old cars for years I can tell you that it's worth taking ten minutes to look at the valves once every twelve months or so, just to stay within tolerances. It's a very simple job that won't even need the cowl on your plane removed (except for the few planes where the engine's completely encased), and it'll make you feel like a real mechanic and impress the pants off your friends at the club who only own 2 strokes when you casually mention "oh I checked the valve lash the other day, it was a few thousandths of an inch out so I tweaked it - runs great now".

Oh and before someone talks you into buying OS because they're "better", my ASP cheapie 4 stroke runs as well as any OS one I've seen, costs half the price, and is tuned exactly the same way (it is after all a direct copy of an older OS engine). Magnum engines are also great value and are identical to ASP's.