The reason Prescott was a 4th round pick was because of off the field issue and becusee he did not look that good against the top defenses in the SEC - look at his record against top teams at mississippi state.
To say Prescott was good against top SEC teams implies you live in Alaska and probably don't have or aren't capable of watching TV. How many top bowl games did mississippi state win with him at quarterback.
Dak Prescott's senior year stats against Alabama - 51% completion percentage, no touchdowns, 1 interception - bad numbers by any standard
Dak Prescsott junior year stats against Alabama - 56% completion percentage 2 touchdowns, 3 interceptions - bad numbers again
For every Tom Brady there are 100 other quarterbacks who were not picked in the top 3 rounds who aren't in the NFL.