Round 1 is 4 years with a team option for a 5th, but the 5th year, while reasonable, is not cheap - for a top 10 pick, it's the same as the transition tag after year 3 of the contract. For the rest of the 1st round, it's the average salary of the 3rd through 25th highest salaries at the same position (who came up with that???). So a team can hold on to a QB for 5 years, but I can't see any forcing a starting QB to do that.

Players chosen in later round all have 4 year contracts, with no club 5th year option.

As pbmax said, the contract can be renegotiated/extended after year 3 - before that, teams/players are not allowed to touch it.