The Novoflex bellows is much smaller and made of plastic-covered material thus stands up to *real* weather. The version I prefer is the one with a built-in rail below the ordinary bellows unit.
The PB-4 would be the nearest equivalent in the Nikon line-up, but it is three times bigger at least. And much heavier. Build and workmanship are superior, but that is of less importance in the field as long as the gear works. The Novoflex by virtue of its petite size can be squeezed into tight places impossible to reach with a bigger bellows.
The bellows draw is longer on the PB-4, but either bellows will allow infinity focus and work in the near range up to approx. 1:2 (and larger, with the PB-4).
A final point concerns adaptation to the camera. The PB-4 is bulky and not entirely comfortable with today's camera sculpted contour, thus for nearly all camera models one has to add some extension to the rear of the bellows. I frequently use the very study E/E2 rings. Only the Df will attach directly without extra extension.
By contrast, as the Novoflex is so small, one often can get the camera attached directly to the bellows rear mount. In some cases the rear mount needs to be rotated 90 degrees to mount the camera, but this rotation is a built-in feature anyway thus no hassle as such.
The main drawback of the Novoflex, for me, is that the controls are designed for right-hand people. I'm left-handed and often lock the bellows instead of focusing it, as the bellows movement knobs are on the right and the locking buttons on the left side.
