A 4-foot-wide opening in a non-bearing partition wall and the same 4-foot opening in an exterior bearing wall are not the same framing problem — the bearing wall needs a header engineered to carry the load stacking down on it; the partition wall generally doesn't.
How to identify a bearing wall
Load-bearing walls typically include all exterior walls, and any interior wall that supports floor or roof framing above it (often identifiable by joists or trusses running perpendicular to the wall and terminating on top of it, or a wall that's stacked directly above another bearing wall on the floor below).
How to identify a non-bearing wall
Non-bearing (partition) walls carry only their own weight — commonly interior walls running parallel to the joists above, or walls that don't align with anything structural above them.
When in doubt, treat it as bearing
The practical risk: if a wall's bearing status is ambiguous from the plans (which happens more often than contractors would like), the safer default is to treat it as bearing and size the header accordingly — a header that's oversized for a non-bearing wall costs a little extra material; a header that's undersized for an actual bearing wall is a structural liability, and one that often doesn't show up until much later.