Consider a capital budgeting problem with six project represented by binary (0 or 1) variables X₁, X₂, X₃, X₄, X₅, X₆.

1. Write a constraint modeling the situation in which at least 2of the project from 1, 2, 4, and 6 must be selected.
2. Write a constraint modeling the situation in which only 2of the projects from 1, 2, 3, and 4 must be selected.
3. Write a constraint modeling the situation project 2 or 6 must be selected, but not both.
4. Write a constraint modeling the situation in which at most 4 projects from the 6 can be selected.