I was trying to do something similar in one of my apps and thought I was doing something wrong, so I'm glad-ish to see someone else with the same problem. I can't find
findAll in any Hubitat docs, but I figured it was a standard Groovy method and I'm not sure if it would even be in there (confession: I haven't used Groovy outside of ST or HE, so this may not be a correct assumption). In any case, I'm sure you're aware of workarounds, but I had to do something like this:
def c4dev = 
if (it.name.contains("C4")) c4dev.add(it)
This should to the same thing, just with a little less syntactic sugar. Note that the above is a totally untested modification of something similar to what I ended up doing in my code, so I hope I didn't make any glaring typos or logic errors, and excuse me if I did.