Mike,
I was recently told by my friends, 2 of them are mechanics, that V6 engines run perfectly for a while and when they break down they cost more to fix than a V8 or a V4. And they aren't too reliable after a while.

This is the kind of question that has no real answer because it is trying to compare apples and oranges. Most V8 engines that have been marketed in the last 10 years are overhead valve push rod engines bred, born and built in the USA, the V6's during the same time are probably an even mixture of push rod and overhead cam engines from every country in the world some very good some very poor. The small block Chevrolet engine has evolved into a very dependable good performing engine, that with fuel injection lasts along time and when it is rebuilt it is by far the most reasonable priced parts wise of any engine in the world and in this same thought most American built V8 engines are good values. The V6(non overhead cam) on the other hand is "almost" as expensive to build as a V8. The overhead cam versions are "much" more expensive as most of these engines are built in Japan and Europe by a multitude of manufacturers. All foreign built engines no matter what the manufacture are twice as expensive as their American counterpart - check out the price to rebuild a 4 cylinder Toyota engine like a 22 RE which is very popular. The 3800 Buick built V6 which was originally designed in 1961 and has probably been sold in more cars then all the other V6's combined, is still reasonably cheap to rebuild, usually goes wellover over 100,000 miles before requiring anything major and therefore disputes the 2 mechanics opinions. As far as I know the V4 that is referred to is one I have no knowledge of as I think SAAB was the only company to market this configuration and I'll guarantee that it is very expensive to maintain let alone rebuild.

Mike