Will the Sims 4 Business & Hobbies EP go on sale during spring sale?

No one doesn't really have definitive information for that here. You'll just have to wait and see.

Usually new packs being on sale I don't usually see for a good few months based on the usual patterns. If it does it probably will only be about 20-25% off. Probably won't be til summer time until you see anything half off for the pack. (Or usually when there will be another major pack after that)