Why? Simple: it's all in the firing order and the number of crankshaft bearings.
A V-4 is an engine hard to balance and has few bearings. You can only have three bearings: one bearing at each end of the crankshaft and one in the middle, between the "pairs" of pistons. That's a poor configuration for high rpm: it doesn't last.
A V-8 is better balanced and has five bearings: it lasts a lot.
Simplifying the "balance" thing: in a V-8 (or even in a V-6) you have more impulses (from each piston) and every impulse has less power, compared with a V-4. You have several firing orders you can choose from, to try to balance the impulse of a piston firing with the impulses of other pistons in compression, admision or exhaust.
Most V-4 and inline 4s must have an axle with counterweights that balances the vibration caused by the pistons firing. You don't (normally) need that kind of tricks to balance the vibrations in a V-8: you can do it with the pistons themselves. In some old V-6s and V-4s, without a balancing axle, you get nasty harmonic vibrations at certain engine speeds (altough CMSMJ1 seems to like the sound, it's not nice to my ears: you can
hear the engine suffering, if you "become" the bearing, zen style! Of course, I can understand your point: when you feel the 65 Hp under your butt, you forget about balancing, but...).
You also have up to five positions for crankshaft bearings in a V-8. Ergo, you can have an engine that you can take to higher RPMs: the crankshaft has a better "support" and the loads are distributed in a better way.
That explains why V-8 engines are so popular: you have several alternatives in the firing order so the running is very smooth and you can have a larger number of bearings, so each one is under less stress.
Inline sixes can also have many crankshaft bearings, that explains why they are a favorite for engine diesels.
I quote: "When designing an engine, choosing an appropriate firing order is critical to minimizing vibration and achieving smooth running, for long engine fatigue life and user comfort, and heavily influences crankshaft design."
For example, in the movie Cars, the neon lights of Flo's V8 Cafe fire in the firing order of a Ford Flathead V-8: americans seem to be really proud of that (or animators in that movie are a curious lot).
See the lights on top of the roofs, left and right? They have a partial answer to your question... 
So, if you have to choose between two cars with the same HP and displacement, choose the one with more pistons (generally). Its engine will last more (or you can take it to higher RPM, if you wish to tune it).
In short: be the bearing, little grasshopper. Ooooommmmmmmmm.....
