Bump version to 0.4

Summary:
Will tag release after merging

**Preliminary Reviewer(s)**: aqiu

**Final Reviewer**:

**Docs impact**:

Test Plan:
go build
./dbbench --version
go test

Reviewers: aqiu

Reviewed By: aqiu

Subscribers: engineering-list

Differential Revision: https://grizzly.internal.memcompute.com/D41257

dbbench.go

@@ -106,7 +106,7 @@ func main() {
 	}
 
 	if *printVersion {
-		fmt.Println("0.3.1")
+		fmt.Println("0.4")
 		return
 	}