N=4 superparticle and super Yang-Mills theory in USp(4) harmonic   superspace
  We study the N=4 harmonic superparticle model, both with and without central charge and quantize it. Since the central charge breaks the U(4) R-symmetry group of the N=4 superalgebra down to USp(4), we consider the superparticle dynamics in N=4 harmonic superspace with USp(4)/(U(1)xU(1)) harmonic variables. We show that the quantization of a massive superparticle with central charge leads to a superfield realization of the N=4 massive vector multiplet in N=4 harmonic superspace. In the massless case without central charge the superparticle quantization reproduces three different multiplets: the N=4 SYM multiplet, the N=4 gravitino multiplet and N=4 supergravity multiplet. The SYM multiplet is described by six analytic superfield strengths with different types of analyticity. We show that these strengths solve the N=4 SYM constraints and can be used for the construction of actions in N=4 harmonic superspace. 
I.L. Buchbinder, O. Lechtenfeld, I.B. Samsonov