I'd go a little further than @s4nchez here. The testability of http4k (really the Server as a Function model TBH) and how it's baked into the fundamentals of how we built software is not just an advantage - its an absurdly underappreciated superpower, affecting everything from reusable test contracts to documentation auto-generated from muti-system test suites.
By way of example, here's an HTTP traffic trace diagram that we generated from a test-case using the TracerBullets that are currently in the incubator module. This is available to all of our tests with no setup other than the Events model and a JUnit annotation