Is 4 ten-feet 4x4s enough for a shed foundation?
4x4x10 (spaced 4'0" apart) is rather questionable and I would expect it to sag in the middle where the shed "runners" (labelled #10 in diag.) rest in the middle of the 10'0" span.  If the shed floor joists (FJs) (#9 in diag.) are attached to the runners (#10) the sagging could be minor.

4x4's are inferior to sistered 2-2x4's because the grain in 4x4s is directionally uniform, whereas, 2-2x4's will have differentiated grain.

I couldn't find a link to a span table for 4x4's but found this one (which recommends 2-2x10's for a 10'0" span): span table  However, this assumes underlayment only and your shed has runners and FJs.  

While I think 4x4x10 is "doable", I think (sistered) 2-2x6's should be a minimum. It really depends on what the live weight load will be (ie. what's going to be stored in the shed...especially toward the center).  In my 6x10 shed I used 2x10x6'0" (@ 24" center) with a 2x12 rim joist.