A year is an integer and even supports integer arithmetic in a meaningful way so text makes no sense. You don’t have a month or day so date is right out the window. The fine manual has this to say about smallint:
The
smallinttype is generally only used if disk space is at a premium.
That leaves integer as a natural choice. If you’re planning to combine this value with dates and timestamps, then interval might make sense as the date and time functions and operators have a good understanding of intervals.
So it depends on what you’re planning on doing with this “year” and what it really is. Sounds like a simple integer is appropriate but you might have to muck around a bit to combine it with your “second year-month-day column” (a date column presumably) depending on how they need to work together.