Does ES 4.02 require .NET Client v4.0.0 ?

We are running .net client 3.9.2

The following blog post in the section “Will I have to upgrade my clients” covers this.
Short answer is no.

Is there any particular reason you ask?

Looks like the event checkpoints/number changed from int to long, so just wasn’t sure if the old clients were still supported.

they will work until you overflow the int32