On any given play, he'll look for WR1, scan for WR2, and if WR3 is open, then he'll consider it.
WR4 almost never gets a look. If WR3 isn't obviously open, he usually takes off running, rather than scan for WR4. He had plenty of time this season, seeing how that offensive line did a nice job of pass blocking for him, especially against that weak competition.
That kind of method will get you killed in the NFL in the same way that Steve Young took way too many hits after becoming the starter in SF.
Someone like Mac Jones will last a lot longer in the NFL, since he tends to stay in the pocket, and if WR4 isn't available, he'll toss the ball away.