I want to build a new quad, but kind of struggling to decide whether to go for 5" or 4". What are the advantages /disadvantages of them?

5" handles better in the wind due to size over 4".
4" are lighter and more agile, and seems to change directions faster/easier and running high kv motors it seems to accelerate out of corners faster as well.
4" is better indoors then 5". There are more prop options for 5".

4" quads are just as fast as 5". I have multiple quads in both sizes and I like racing the 4" ones more. Not arguing that a 5" shouldn't be faster in theory, but in practice I'm faster around the track on 4".

5" is hands down the best compromise between power and efficiency. 4" is more of a novelty at this point.
Also, after seeing only 5" on the biggest track ever I'd say it's safe to say 6" is also dead. lol.

Thanks guys. I got the chance to fly a good 4" setup for a while this past weekend and was really surprised at the agility. And the overall power was really strong. We were flying a short course, and I didn't find that the 4" was any slower.

Definitely noticed the 5" as more efficient with flight time. Definitely liked the 4" setups willingness to drop altitude quicker - probably a weight vs. prop size correlation there.

Overall, I could see adding a 4" setup some day, the agility is impressive.

I noticed the biggest difference in the low throttle region, since obviously the 4" setups need more rpm to behave the same. To counter that i need a lot of negative expo on throttle curve to boost the low end.

And 4" has especially less grip on catching after drops, at least that's how i destroyed mine...

Get some high kV motors!