A good snapshot of overall performance can be had at 8000', wide-open throttle (which is kind of a nominal 75% cruise configuration). The advertised performance numbers for the -4 on the Van's website show 192 MPH TAS under those conditions with a 160 HP engine.
Things do depend on how well your pitot/static system is calibrated, but some GPS data runs under those conditions (or adjusting altitude to approximate the 8K', standard condition) will give you a reasonable ball park comparison so that you can do a real apples/apples assessment without doing too much extra testing.
There's quite a bit of good info over on Kevin Horton's site on how to dial in the pitot/static system, and lots of different spreadsheets that can provide good data based on four- or three- leg GPS runs.
By way of comparison, here are the numbers for our very-stock -4 turning a 2-blade 68 x 72 Gen 2 Catto at different altitudes and conditions. All of the data is the result of test and have been corrected for conditions, so it might provide some insight into conditions other that 75% cruise for a stock airplane with a similar engine and fairly close propeller combination.
The bottom line is that you should be pretty close to the numbers published under the "Performance" tab on the Van's webpage.
Please give due consideration to how fast you fly, as structural margin is reduced at speeds above Vno (180 MPH IAS for the RV-4) and and flutter margin is reduced above Vne (210 MPH for the RV-4) which is based on true, not indicated, airspeed.
Good luck and feel to free to drop a line if you have any questions or would like a spreadsheet that might help crunch some of your airspeed flight test data.