Should a 4-Person Startup Build a Formal Design System?

A 4-person startup (one designer, three engineers) building a single B2B product asks for your advice. They've just hired their second designer, and are debating whether to spend the next sprint building "a proper design system with full documentation, versioning, and a component contribution process" before starting any new feature work.

1. Would you recommend they build the full governed system now? Justify your answer using the tradeoff this subject describes.
2. What signal, if it showed up in the next month, would change your recommendation?
3. What's a lighter-weight alternative that captures most of the near-term benefit without the full governance investment?