The F4 is a very good and capable detector. It can go as deep as some of the $800-$1000 machines and can discriminate right there with the best of them.
The stock coil is what limits it from being really deep.
Take a better look at something like the Teknetics Omega...or its brother, the Fisher F5. The Omega is a bit better IMO and with a 13" DD coil....it's deeper than just about any VLF machine I've ever ran.