Most ABC and ABN engines can be four cycled with no ill effect. In fact many of the control line stunt engine are ABC and ABN engines that are four cycled when normally flown. Dub Jet himself makes them. Only engines with an extreme taper, or a very tight fit have problems with this and usually only when first broken in. The tight fit may cause them to grab the piston and break a rod when richened to a four cycle. If you follow OS instructions on many of their engines to the letter you will break them in at a four cycle without having to listen for the sound. The instructions will tell you how far to richen the needle from peak and many of them will break into a four cycle at that setting. No problem.