4-way stretch means the fabric stretches both horizontally and vertically and bounces back to its original shape. For skiing, this matters because you're constantly bending, twisting, and reaching—whether you're carving aggressive turns, hiking to fresh lines, or tweaking grabs in the park. With 4-way stretch, your base layer moves with you instead of fighting you.
Most 4-way stretch base layers include a small percentage of elastane (also called spandex)—even just 2% makes a noticeable difference. The real benefit beyond mobility is fit retention: the garment snaps back into shape all day instead of sagging or bagging out by your last run.
If you're a casual resort skier who mostly cruises groomers, 2-way stretch is probably fine. But if you're skiing aggressively, hitting the park, or just hate feeling restricted in your layers, 4-way stretch is absolutely worth it.