I think we are using the word "difficulty" in two different senses. I just mean, what is hard to accomplish, rather than "what is the ISU base value." A really spectacular full Russian split jump has no zero base value, but it is hard to achieve even so. (However, a mediocre one is easy, as we see in almost every program.)
[Dick Button soars above Central Park in the 1950s. T oo bad you can't actually point your toes in a skating boot.
https://i.pinimg.com/originals/7e/cf/7d/7ecf7dfe485e9da657d1e7d408d3b2df.jpg ]
As for the other meaning of "difficult element" -- a triple Lutz, regardless of quality, is inherently more difficult to perform than a triple toe loop, a quad is more difficult than a triple. etc. -- that's true enough.
But then, too, it seems to be really, really hard to do a quad loop (at least only one or two men in the world can make a serious attempt at one), whereas quad Lutzes are becoming commonplace, even for 14-year old girls. I am always surprised anew that the first triple jump was not an "easy" toe or Salchow, but a "difficult" loop jump. Maybe for some reason it it wasn't so difficult for Dick Button back then.]
So "difficulty" is relative to the skater i guess. ISU should maybe re-work the table of points per element then. Or even maybe erase the BV concept.
About 4lz. It's probably the most cheated jump now.