Petite sizing is designed for people 5'4" and under. The numeric size (4P, 8P) refers to the same waist/bust measurements as regular — only the length is shorter.
Differences: petite inseams are 2–3" shorter, sleeves 1–2" shorter, jacket bodies 1.5" shorter, rise on pants is shorter.
If you're 5'4" or shorter and frequently have to hem pants or roll sleeves, switch to petite. The difference is dramatic.
Major petite lines: Ann Taylor Petites, J.Crew Petites, Banana Republic Petites, ASOS Petite, Loft Petites.
Petite ≠ small. You can be plus-size and petite. Some brands offer both Petite Plus and Petite Regular.