Why a 4×4 scramble looks different
The 4×4 has no fixed centre pieces: each face has four centre pieces that move freely, plus pairs of edges that can be split apart. A scramble therefore has to mix the inner layers as well as the outer faces, which is what the wide moves are for.
How the 4×4 scramble is generated
cubing.js generates 4×4 scrambles from a randomly chosen position, the same principle as the 3×3, rather than from a list of random turns. The sequences are longer, around 45 moves, because they have to place the centres and pair up the edges as well as the corners.
A side effect worth knowing: because the position is truly random, parity cases come up as often as they do in real solves. If your 4×4 method has a parity step, these scrambles will make you practise it.
Reading wide moves
Outer-face letters work as on a 3×3. A w adds the layer next to that face: Rw turns the right two layers together, Uw the top two and Fw the front two, with ' and 2 meaning counter-clockwise and half turn as usual. Start from a solved cube with white on top and green in front.
45 moves is a lot to apply by hand. Read the scramble in chunks of four or five moves, and compare with the preview when you finish.
Practising the 4×4
Most cubers solve the 4×4 by reduction: build the centres, pair the edges, then finish it like a 3×3. Time full solves with real scrambles, and note which stage costs you the most; that is usually the one to practise.