In all honesty most 4th rounders never amount to much. I think only about 1 in 5 ever develop into decent starters. At this point if you can find a significant contributor at something besides ST you have done well.
These are all develomental picks for the most part because the players usually have some significant shortcomings and that is why they go here. Maybe it is lack of size, strength, speed, quickness, experience, injury history, toughness, intelligence.... But there is something that made people wait this long before calling them on down.
So if you grade someone as the best for your systems you go ahead and take the player and worry about other things afterwards. You can usually still try and fill some depth gaps in what is leftover in FA. Here is where you draft for 2-3 years down the road and needs can change markedly due to injury, age or salary in that timeframe.