Sigmafy the expression
Mathematica, 17 14 13 bytes

Thanks to Ian Miller for saving 3 bytes.

Thanks to LegionMammal978 for saving 1 byte.

#~NSum~{k,5}&amp;


The input should be an actual expression containing k, e.g.:

#~NSum~{k,5}&amp;[Sqrt[k]^3+4]