There's nothing wrong with the original here. "You can use up to 4 craft in a combat" is perfectly acceptable use of grammar. It intones that you can only use 4 craft in any one instance of combat. Which is what it's trying to say. Saying "You can use up to 4 craft in combat" or " You can use up to 4 craft for hijacking/interceptions" suggests that you can only use 4 anywhere on the globe at any time.
Kinda weird to have others double-checking the reports but uh ok.
Anyway,
X-COM manual (page 78 in manual, page 79 in .pdf reader):
Note: You can also use the Minimize Window icon to give other Interception Craft
orders to attack the UFO. There can be up to four Interception Craft attacking a
UFO simultaneously, maximizing the chances of success against the larger UFOs
So, yes the original can technically work but it reads abruptly; removing the 'a' makes the text read more clearly. In addition, what is "a combat"? It's mostly understandable but again, it's not 100% clear. It could also be misunderstood as being able to use 4 transporters for a single BattleScape; instead of just interceptions.
Lastly, considering the entirety of the entry is:
Warships should be attacked by multiple craft.{NEWLINE}You can use up to 4 craft in a combat.
I can understand your point about misunderstanding that only 4 can be used at a time [anywhere, on the globe, anytime]; but as the entry suggests attacking larger ships with multiple craft, it's a far reach. However, translations in other languages could alter that sentence structure to fit your logic.
I would have suggested "..., simultaneously." like in the X-COM manual excerpt above, but not trying to recommend huge sweeping changes; just small tweaks for readability. The [hijackings/interceptions] bit is editorial shorthand instead of having to write: "You can use up to 4 craft for hijackings." & "You can use up to 4 craft for interceptions." Also, to offer a "flavor" suggestion and a "standardized" suggestion.
Anyway... it's all up to Dioxine, they are only suggestions.