The runt of the litter, Low Horn 4 is not only the lowest of the horns, it also is barely more than a scramble up from the north. It is, however, the best way to get to Low Horn 5, which is quite another matter.
The route up Low Horn 4 is well enough visible as well as obvious looking at it from the saddle. Generally, it goes a bit to the right of the ridge line, later right on it. The crux, such as it is, is a bulgy section about a third of the way up. The holds don't feel that secure and there is exposure as well. It may be a place to rope up for.
As with all of the Low Horns, the easiest route up is from the north. Lichen-covered though they are, the north faces are generally much less technical than the south faces.