I'm not going to say it's categorically impossible... but total conquest in 40 years from a Catholic count/duke-level start seems improbable to me. Two reasons.
1. Recently conquered penalties. They would make stuff like Holy Wars, Crusades, and Invasions far less rewarding if you're trying to compound over just a 40-year window. Conquered new territory? Great. But now you have to wait half the length of your game before it starts contributing, unless you change religions or it gets converted first. Similarly, you simply can't count on having heirs with religions / claims you need - it's entirely possible your starting ruler will live to age 80 or more, and you're stuck with him the whole time.
2. Existence of claimants. Until a starting ruler dies, most titles have no claimants. You can force claimants to be generated with an assassination, but... that will get very expensive, very fast. Even after one ruler has died, it's not uncommon to have no workable claimants for a title; it's not until after a few generations have passed that you can rely on finding claimants for titles.