L4 in the Vail/Keystone description is green only, L5 is greens and groomed blues. But keep in mind that a green at Keystone may be more like a blue at small ski areas in flatland regions. My daughter was L5 (of 9) at Alta ski school at age 7 during her first ski trip out west when she was skiing blacks all over the southeast. She was skiing Alta blue groomers without any problem. Her instructors at Mnut as a beginner/adv. beginner were often young adults who were intermediate skiers who had never skied anywhere but VA. Some has passed PSIA Level 1 or were planning to take the exam at the end of the season. At Alta, she had PSIA Level 3 instructors with 15+ years of experience (usually in April). In short, there certainly are regional differences as well as differences between small local ski areas and destination resorts.
I observed a PSIA clinic for the Children's Cert at Massanutten a bit a few years ago. The trainer often asked the instructors he was training act to like kids. It was a mixed group. Trainer was on a snowboard and the students were on skis or snowboards. He made it a lot of fun. Key point was that lessons for kids should emphasize the fun.
When I started doing an adults-only ski week out west, I was pretty adventurous when it came to ungroomed terrain. That was in 2010 in north Tahoe. Since I learned survival skills like side slipping and kick turns long ago (straight skis), was confident I could get down pretty much anything that wasn't too narrow. Could follow someone in open trees or on a bump run but not likely to put together more than 10 turns before needing a rest. As far as technique went, I was probably Level 6 more than Level 7. Even so, I was in the lower advanced group for the Diva clinic instead of the intermediate group, which stayed more on groomers. By the end of the next season, I would say I was definitely Level 7 (of 9). That meant I was more willing to go with friends who were better skiers on Ballroom or Sunspot at Alta. Took another season or two before I was willing to head around to the Backside or out to Catherine's in good visibility.
...No, seriously, because I've been looking for a clinic for this year. ( Privates are another alternative--thanks, Divas--but if I'm travelling alone the cost gets prohibitive pretty fast). 
You remind me of another Diva. It took her a while to figure out her learning style. Once she did, she found a couple Level 3 instructors who she really liked. For her, multi-week group lessons did not work at all even as an intermediate....Because I'm bored at work today and ski season seems so far away
Totally agree with you as to terrain, technique, and learning style. I don't love off piste, but I don't stay on blue groomers either. I'm comfortable on a black run, as I said with no points for style, and I'll make it down a double black but it won't be pretty. I've spent a lot of time on skiis but have had relatively few lessons, so the techniques/drills/exercises are usually new to me.
The learning style thing is huge. What do you call it when you can't learn by just hearing instructions? I need to hear what I'm supposed to do, understand why I'm doing it, watch someone else do it , try it myself, review why I'm doing it again, and keep trying. I don't do well with instructors who give me an exercise ( which I usually don't understand and so don't execute properly) and then ask "What did you feel?" I need to begin with the end in mind. I'm strong on analytical skills and weak on physical coordination and spatial orientation--so I must be compensating. But I don't know how that translates to "learning style."
FWIW, my other sport is horseback riding and I had exactly the same issues--can stay on anything, zero technique. What it took to was well-trained horses ( the analogy would be the right equipment) , private lessons weekly with a world class instructor, and a whole lot of work--I basically went back to square one and relearned to ride.
The more I think about it, the more I'm thinking private lessons and lots of them....with an instructor who suits my "learning style", once I figure out what it is!
