Welcome to the community!
Tomorrow I'm helping a buddy check out in his new to him -4--he just bought it and hasn't flown it yet. He is a test pilot school grad that currently works in the flight test department for one of the major airframe manufacturers. We lost a squadron mate about 15 years ago when his jet came apart during a simple captive weapons carry test due to flutter.
I'm not aware of any Van's mishaps due to flutter, although there have been several structural failures over the years, most of which were due to pilot handling errors. Proper flutter testing is actually a bit of a black art and has already been nicely described in this thread. A couple of things happen when you start to go fast: first, you have lots of aerodynamic G available (more than enough to pull the tail off the airplane if you pull the stick hard enough and fast enough); and second, if you exceed design limits you have a diminishing flutter margin. If you exceed design limits enough, you may have no flutter margin, but just exactly were that point is is unique for your particular airplane. Interestingly, the point at which flutter will occur for a particular airframe will actually change over time with damage or wear. Vno (maximum structural cruising speed) for the RV-4 is 180 MPH indicated airspeed and should only be exceeded in smooth air. Vne (never exceed speed) is 210 MPH and is, effectively, a true airspeed. So if you go high enough, 210 MPH true can occur at indicated airspeeds less than Vno. It's also important to understand the effect of gust loading on the airplane at speeds in excess of Vno. It is certainly possible to generate 200 MPH TAS at altitude in the -4 in level flight, and quite easy to exceed that in a descent.
I put RV-specific aerodynamic discussion, including flutter, in the training syllabus on the safety page that you might find helpful. You can download a copy if you'd like via the hyperlink in the most recent post in the thread in the sticky section. The table of contents is hyperlinked to help you navigate the information. If you have any questions, you can post, drop me a PM or email and I'll be happy to answer them. And I'll parrot what Dan said--there is quite a bit of discussion on this site and in the RVator that you can dig through using the search function that will be very helpful as you transition to your new plane.
I use my O-320 powered airplane to commute to work and cruise between 7000-10000 feet at 180 MPH TAS which reliably provides 150 knots ground speed at 60-65% power. I posted a copy of the handbook for my airplane on the POH tab and there are several others there for RV-4's that you may find helpful as well.
You're going to enjoy your new airplane but might find that it takes a while for the grin to wear off after that first flight!
Cheers,
Vac
But seriously, I think you'll find your average cruise to be more in the 160 mph range depending on how you run it. The -4 is a great flying machine and once you get acclimated to its performance, both fast and slow, I have no doubt you'll enjoy it.
