You've overvaluing the 4th round pick for the other positions and undervaluing it for the QB position. That's really the bottom line. Only the QB moves the Vegas line more than one point if the starter is down. We have plenty of depth at other positions and I got news for you, a fourth rounder isn't likely to be a starter or even be a multi-year contributor. Look over the list of 4th round players we've had in the last 20 years. Only ONE guy has started and contributed more than one year. LL tweeted that yesterday. Since you're not asking the QB to come in very often, he'll be sticking around for sure which gives us a cheap way to carry a backup instead of constantly paying Grossman 1 million a year or giving another vet a multi-year contract.
Also the round a guy is drafted in has nothing to do with how much you get in return. Its all about how he performs in the chances he has. Look at Matt Schaub. Texans swapped firsts with ATL and then gave ATL two succeeding second rounders. Schaub was drafted in the third.