Wedding etiquette apparently dictates black tie and evening dress for a 4pm wedding. Vogue says if a wedding is taking place at 4pm or 5pm, you should "wear something that easily transitions from day to night".

We’re not going with conventional wisdom on this though, so it’s "sharp suits and pretty frocks". In practice, just go with whatever you feel comfortable in - dresses, skirts, trouser-suits, dungarees, etc. Ties entirely optional (dodgy pun intended)...