  We investigate the D0-D4-brane system for different B-field backgrounds
including the small instanton singularity in noncommutative SYM theory. We
discuss the excitation spectrum of the unstable state as well as for the BPS
D0-D4 bound state. We compute the tachyon potential which reproduces the
complete mass defect. The relevant degrees of freedom are the massless (4,4)
strings. Both results are in contrast with existing string field theory
calculations. The excitation spectrum of the small instanton is found to be
equal to the excitation spectrum of the fluxon solution on R^2_theta x R which
we trace back to T-duality. For the effective theory of the (0,0) string
excitations we obtain a BFSS matrix model. The number of states in the
instanton background changes significantly when the B-field becomes self-dual.
This leads us to the proposal of the existence of a phase transition or cross
over at self-dual B-field.
