I have no doubt that there are some 5s around that are "OK" and feel almost like a 4. But I'm sorry, why would anyone stop playing their lovely 4 (with or without extension) and play a 5? To sit in their section thinking " this is almost as nice, almost as easy to play as my 4"??? Not for me, I'm afraid. The Barnaby extension works beautifully, the bottom end sounds great, an when I don't need it, it goes away. I know there seems to be a fashion growing for 5s, and I can play them. HOWEVER I don't want to, and I don't have to. I do this for enjoyment these days, and playing a 5 is less enjoyable than playing a 4
I prefer to play a five string bass.
I bought my main bass ca. 10 years ago as a four string bass. I wanted a five string bass, but couldn't find one in my price range that was a least OK. 1 year later I had a C extension made for it. It was nice for a lot of music, but for jazz/improvised music playing it was very inconvenient. There is just not enough time to prepare the capos, if you want to play a low note suddenly. Also I prefer the sound of a very fat low B string when playing pizz. Somehow it sounds/feels more bassy than the same notes on an extension.
Today I'm mostly playing contemporary music, where a fiver is much more usable than an extension, because composers are writing for it specifically. At least in Europe.
So one or two years ago I had the exact same bass rebuilt as a five stringer and couldn't be happier. It took some time of setting it up well, but it can be done.
When comparing four stringers to fiver stringers, always keep in mind, all the other factors of the instrument. There are many small 3/4 or 5/8 basses around, that are a joy to play solo rep or jazz on. Of course most five stringers are harder to play, but that's very often because they are much bigger. Big (often too big) shoulders, large (often too large) rib depth, long (often too long) string length. And may of them sound like cardboard, because of redicously heavy construction.
It's much fairer to compare well made, well set up four strings to well made, well set up five strings. Many fivers you can find in schools, universities and even orchestras don't fall into this category.