2nd rounders have no bargaining power, if the team will only offer them 4 years then they will have to take it. For foreign players it is harder because I suspect they are more than willing to spend time in their home leagues, but most Americans would rather play in the NBA I imagine.
If you want to get someone to sign for 4 years you'll likely have to guarantee the first two which is what happened with Parsons, a risk a lot of teams probably don't want to take. However, a first rounder is guaranteed the first two years also so it's not a massive risk if you're trading down a couple spots in to the 2nd round.
Personally, I don't like the idea of trading #30 for #39 and #45.