Quick answer: The best time to visit Medellín is December–March (drier); spring-like all year. Expect a mid-range budget of about €45 per day. Visa: Visa-free for most EU/US/UK passports (90 days, extendable).
Why this window
The city of eternal spring is a nomad hub for a reason. Four days: El Poblado cafés, Comuna 13's transformation, Guatapé's rock and lakes.