Careful what information you ask for on the internet...
There is no such thing as being too fast on top of a loop.
This is ONLY true provided it's flown as a round loop, (which you do allude to) and initial stick pull force (resulting G forces) are the same as the exit stick pull force (and resulting G force). The alternate is pulling hard at the beginning (resulting in higher airspeed over the top) and lazily pulling on the back side. Now you're easily "too fast" over the top for that type of loop. Airplanes have a lot of gray areas...
Also, GHARBEN, not to be rude and assume, but based on previous postings, you're new to the RV-4? RVs in general? I dont' know your aerobatic background, but if it is minimal, PLEASE find someone knowledgeable near you and get some instruction prior to venturing too far on your own in a new airplane. That's when the holes in the swiss cheese model line up, my friend.
This discussion would benefit from some specifics. This by no means intended to be instructional information. Just clarifying what others have added. There are MANY ways to "loop" and airplane... some better than others.
A perfect, round loop begins and ends with the airspeed, altitude, and heading exactly the same, making a perfect vertical circle in the sky. To do this, you must pull the same G forces in the first quarter of the loop as the last quarter of the loop. Your stick position and pull forces should constantly be changing with regards to airspeed.
Relax the G forces too early, and you create and "egg shaped" loop by adding too much vertical component, too low airspeed, and sometimes "flopping" over the top. Correctable by delaying your recovery pull and then you might end up on airspeed and altitude. Correct results, poor execution.
Fail to relax the G forces on the upward portion of the loop and you've tightened it too much. Results would be excess airspeed and not enough vertical component. Correctable by pulling the same excess G forces earlier than normal and ending on the correct airspeed and altitude. Again, correct results, poor execution.
Another thing to address is the idea of the physics. In a perfect world, yes... No need to touch the throttle. Energy removed by gravity going up the loop is added again on the backside of the loop. But we have propellors producing thrust. Whats happening is the engine and prop are fighting gravity on the way up creating a higher potential energy (read: Altitude) than would have otherwise happened in a glider. The same occurs but in reverse on the backside. The engine/prop combo is ADDING energy to the already higher potential energy (read: altitude) on the way down creating MORE kinetic energy than you started with (read: airspeed).
Bottom line? You must pull the power back to make a perfectly round loop and come out on airspeed. RV's (as we know) are slippery and build up airspeed very fast when Gravity assists.
Alternatives to pulling the power? Pull harder on the backside of the loop to keep the radius... Not my first choice, especially if you were fast at the top to begin with.
So to finally SIMPLY answer the OP's question, you CAN be too fast on the top of a loop (or any vertical maneuver) if you don't recover correctly.
My personal max in my 160hp, FP, RV-4 are about 100mph over the top. That is for any vertical maneuver. Split S' also come to mind. If I see 110mph while inverted and beginning to pull, I know I should onset the G's sooner than later or else it will be VERY fast coming out of the bottom.
It should also be noted that these are for 3-3.5G maneuvers. You can pull up to 6G and that will bleed the gravitational energy faster (load up the wing and manage airspeed), no doubt. But I personally don't fly that hard in my -4.
Comments such as "no such thing as too fast on the top of a loop" are dangerous things to suggest with an already slippery airplane. Reference: Smoky's story about Vne being TAS versus IAS. I digress... Please, manage your energies well and keep it within the limits, friends.