all sarms can be ran solo but they are all better when stacked... they feed off each other with a very strong synergy and become far more effective when stacked however when solo, they still provide excellent results...
s4 is well known for lean muscle, muscle hardening and strength but it would not be as strong as s23, which is essentially a steroid even though it is considered a sarm, it has the strength of a steroid... s4 is one of my favorites but i have put out so much info on it, that you can easily watch and read and learn everything about it