2 of those 4 are unlikely to be drafted and Johnson went 161st overall. 278 Gophers have been drafted and 152 of them before Johnson. Some of that, especially for Mo, is their style of play not matching with the current style NFL teams are looking for but the majority of those 152 are on the same level as those 4.
If you look at All Americans, 73 Gophers have been first team All Americans which Morgan, Johnson, and Mo never achieved.
Records are also misleading because the game has changed so much over time. If you compare the 1960 Gophers to these guys, some of the 1960 team such as Sandy Stephens are certainly on the level of the players you mentioned. In 1960 Stephens only attempted 62 passes, this year even with missing 4 full games and parts of others with injuries Morgan attempted 107. In 1960 the top 2 rushers (including Stephens) ran the ball 168 times combined, this year Mo alone ran the ball 320 times. Not to take anything away from Mo and Morgan, they had great careers and deserve to be honored, but there are a lot more players at that level that aren't thought of as much because of when they played and the position they played. I would guess few of even the most dedicated Gopher Football fans could name every Gopher in the college football hall of fame and the majority probably wouldn't even recognize every name.