Expressing forms as a sum of pfaffians

Let A= (a_ij) be a symmetric non-negative integer 2k x 2k matrix. A is homogeneous if a_ij + a_kl=a_il + a_kj for any choice of the four indexes. Let A be a homogeneous matrix and let F be a emph{general} form in C[x_1, ..., x_n] with 2 deg(F) = trace(A). We look for the least integer, s(A), so that F = pfaff(M_1) + ... + pfaff(M_s(A)), where the M_i = (F^i_lm) are 2k x 2k skew-symmetric matrices of forms with degree matrix A. We consider this problem for n= 4 and we prove that s(A) < k+1 for all A.