Your not going to get accurate with 3-4 skin folds. Body compositions varies greatly based on a persons hormonal status and 3-4 skin folds is not going to catch any nuances at all.
There are different formulas the applications and web sites use and they will vary slightly but at the end of the day their general inaccuracy isnt because of the math its because yout not giving it enough source data to accurately compute bodyfat.