4K is a very high resolution, cramming 3,840 x 2,160 pixels into the display. That's certainly true to a point, and some would argue that 4K on a smaller – say 13-inch – laptop probably isn't worth it.
A 4K display is no more likely to cause eyestrain than a 1080p alternative. In fact, it offers greater clarity, which is an asset.
Eyestrain can still occur, but if it does, it's because of excessive brightness, improper positioning, or other factors, not the resolution.