You can purchase a manual for this machine from the following link for $US14.99 http://www.sewingpartsonline.com/instruction-manual-simplicity-sl4300.aspx
I can't see it anywhere for free download unfortunately. But you may find that looking at a manual for another 4 thread overlocker may get you started. If you try this link http://www.singerco.com/accessories/instruction-manuals
you can look at the free Singer downloads,
try this link for their 654 3/4 thread model http://www.singerco.com/uploads/download/555_14sh644_654-(e_s_f)-(new-led-version).pdf
it may give you some help now.
Essentially, to change the stitch width you can either move the lower fixed blade wider to cut less fabric or tighten up the upper and lower looper threads in order to close up the stitching.
Moving the blade usually means opening up the front cover door, putting pressure on the upper blade and swinging it up out of the way. Then you can adjust the lower blade, usally by turning a knob near to it. Sometimes this knob will be marked 1, 2, 3.
Adjusting the looper tensions is done by changing the tension dial settings for the thread needing adjusting. If its a standard four thread overlocker that produces a "marrow" seam it will have two needles and two loopers, upper and lower. Generally on most overlockers, the upper looper will be the second from right tension dial and lower looper will be right most tension dial - turn up one half number to tighten, or down one half number increment to loosen, then test stitch again. The needles shouldn't need adjusting too often.
I usually start with all tension dials on 5, test stitch, then adjust from there. Some fabrics will tunnel (pull in) if the seam is too wide so you need to either move the blade to the left and trim more fabric or loosen off the loopers. Or in the reverse, like you are getting, the threads will hang off the edge.
There is some good serger information on Debbis Cosgrove's pages www.sewing.about.com
but if you have never used a serger, then getting the genuine manual is probably best.