Thanks Sitesurf. To clarify further, without "à", can it technically only refer to duration, or can it also properly be used to refer to the clock time?
(I would have assumed the former, but I noticed it suggested elsewhere that it could have either meaning, so I've come to the trusted expert.)
Sometimes, I need to be careful with the way some of my compatriots speak our language. I wish I could tell you "à" is needed to announce clock time, but I know some don't bother. So the bad news is that you can expect a degree of ambiguity and should be prepared to ask "dans les quatre prochaines heures ?" or "du matin ou de l'après-midi ?" or similar clarification.
- vous y serez en quatre heures = it will take you four hours to get there
- vous y serez d'ici quatre heures = you will be there within four hours
- vous y serez dans quatre heures = you will be there in four hours
- Marseille est à quatre heures d'ici/de Paris en TGV = Marseilles is 4 hours from here/Paris in high speed train
"Vous y serez / Vous serez là-bas pour quatre heures", is ambiguous because it can mean "by 4 o'clock" or "during a period of four hours".
Considering the meaning of "for four hours" as "during a period of time of four hours", you may have to change the verb to: "Vous y resterez/Vous resterez là-bas pendant quatre heures".