My question is regarding the use of the hyphen in a phrase such as “four to five-hour workload”. Is this correct?
Very good question! A major use of hyphens is to link two or more words into a single unit, usually to modify some other word or phrase. In this case it's not a "four" workload that's also an "hour" workload—it's a four-hour workload. The hyphenated words act as a single compound adjective for "workload."
But in this case the modifier is more complicated: the workload ranges between four and five hours. It would be wordy and repetitious to say "four-hour to five-hour workload." So we use a suspended hyphen to indicate we've left out a word:
four- to five-hour workload
The suspended hyphen is a signal: You're not getting the complete compound adjective, but you'll understand it before you reach the noun it's modifying. Note that the suspended hyphen must have a blank space after it. Here are a few more examples:
We heard pro- and anti-war arguments.
He was a major figure in 18th- and 19th-century philosophy.
English- and Chinese-speaking delegates had trouble with the simultaneous translation of the speeches.
While I'm ranting about hyphens, I'll comment on when not to use them: In compound modifiers that contain an "ly" adverb. So it's fine to write
a high-powered engine
but if a perfume has a strong scent, write
a highly scented perfume
For the same reason, remember to write
a barely noticeable noise
a seemingly insoluble problem
—and save your hyphens for when you really need them!