Converting a flying RV-4 to a "proper" Rocket would require a complete rebuild resulting in very few parts resembling the original. Frankenstein would look good standing next to it after completion The Rocket is a completely different airplane physically with a shorter RV4 wing retaining the same number of ribs, a longer fuselage using .040 extensively. You can install the HR2 fast back with canopy as Dave Anders did however, you are better off selling your -4 and building or buying a Rocket.
The original Rocket was a vision of John Harmon brought to fruition in the early 1990's after wanting "more" from his RV4. He sketched the original on the hangar floor with chalk, and set to work building it from scratch. The prototype has over 2000 hours on it now and has spawned another 130 flying examples, mine being the seventh to slip the surlies. My wife and I had dinner with John earlier this year and he still grins talking about the first flight in the HR2. It is rumored that Van, after seeing the HR2 prototype smiled broadly. Three weeks later he had sketched the first RV-8 drawings. Having flown both aircraft extensively I can assure you, there is no comparison. The Harmon Rocket wins hands down.
To build one requires two kits, RV4 from Van's and the Harmon kit. Order the RV4 minus the finishing kit. The Harmon kit comes with plans showing you how to use some of the existing RV4 parts in addition to new parts in the Harmon kit to produce the best flying sportplane* on the planet.
The Rocket is a completely different airplane physically with a shorter RV4 wing retaining the same number of ribs, a longer fuselage using .040 extensively. You can install the HR2 fast back with canopy as Dave Anders did however, you are better off selling your -4 and building or buying a Rocket. 
