N=4 Super Yang-Mills Low-Energy Effective Action at Three and Four Loops
  We investigate the low-energy effective action in N=4 super Yang-Mills theory with gauge group SU(n) spontaneously broken down to its maximal torus. Using harmonic superspace technique we prove an absence of any three- and four-loop corrections to non-holomorphic effective potential depending on N=2 superfield strengths. A mechanism responsible for vanishing arbitrary loop corrections to low-energy effective action is discussed. 
I.L. Buchbinder, A.Yu. Petrov