RV10 pilot reports a NMAC with a Cessna at 4;500 FT.

Narrative

My RV-10 is in the Phase 1 test flight period. I had just completed a series of stall tests in my test area. I was north-northwest bound proceeding back to my home airport and just entering an area known as the southeast practice area. I was hand flying in level cruise flight at 4;500 FT MSL and monitoring the practice area frequency. There were no radio calls and traffic was light as it was a holiday weekend. My workload was light as I was finished with my test plan. All my test scripts and data were set aside - meaning no distractions. It seemed like I had the sky to myself until I scanned from my left to my two o'clock and slightly high to see an imminent collision with a white/red Cessna already taking evasive action by pulling up and turning right. I immediately pushed my control stick over into an aggressive dive. I could feel myself straining against my belts and a tool box in the baggage area became airborne and struck the baggage bulkhead. If no evasive action had been taken I believe the 12 O'clock low position of the Cessna would have impacted the 2 O'clock high position of my RV-10. I don't believe that distraction was a factor in my cockpit as I had completed the flight test script. The EFIS; GPS and auto pilot were already set and I was hand flying the airplane. I think I simply let my guard down and lost the discipline of scanning for traffic. You sort of think the hard work is over when you finish a test script but you can't relax until the airplane is back on the ground.