Been researching this for the better part of the day and can't determine. I understand 4"-centerset is for 4" only, 4"-mini-widespread applies to both 4" and 8" while 8"-widespread is supposed to be 8" only. For the life of me though I can't determine why a widespread faucet could not be mounted on 4" centers. It seems most (if not all) of the widespread bathroom sink hardware includes a flexible hose connection between the valves and spout which should allow for any spacing up to 8". There must be something I'm not seeing. Maybe the above sink hardware is not properly sized for 4" and would interfere with each other? Hopefully someone will enlighten me.
I have a relatively small pedestal sink with 4" centers but like the look and easier to clean aspect of the three piece faucet. Also, I have found a style I really have my eye on and it's available only in 8" with the added bonus of not carrying the premium mini-spreads seem to have.