On a 4x DVD writer, 4x is a ZCLV speed (usually!) - on an 8x or above writer, 4x will be a CLV speed.
To get anything other than basic speed/strategy, you must be using media with a code recognised by the writer (updating firmware may help) - the way some media rebranders use various codes with little distinction doesn't help, and especially in +R media, there is the version number as well.
4x media is more likely to be supported at 4x by older drives, unless the maker is really serious about adding recognition for 8x media at 4x.
So Zone-CLV is the more conservative strategy, and P-CAV is the faster one.
There seems to be a transition in progress in DVD writers, while CD-R seems to be almost exclusivly P-CAV now - a few of the Zone-CLV DVD writers also do CD-R that way.
In CD-RW (Ultra, 24x), Zone-CLV still has a very strong presence, maybe to do with the particular calibration requirements. There are some P-CAV drives, but they are very much the exception.
The other issue, perhaps CD-RW is fading in importance, as DVD rewritable has it outcapacitied and outspeeded. It's still useful for testing, of course.