This is the fourth article in a series describing efforts to produce tough, high-performance thermosets from very low viscosity prepolymers which are autoclave processable. Hydroxy-terminated hyperbranched polyester (HBP) with a systematically increased molar mass was used to toughen bismaleimide (BMI). HBP was dissolved in the allyl phenol component, B, of a two-part BMI, to yield homogeneous solutions. The BMT monomer, A, was dissolved in the solution of HBP in B to give homogeneous prepolymers. The fracture toughness (K-Ic) of neat resin plaques was measured by compact tension, while the T-g and storage moduli (E＇, at 55 and 200 degrees C) were determined by DMA. At 9% loading, the K-Ic of the BMI increased steadily with HBP molecular weight up to 138% over the control with G5 HBP (M-n similar to 14,000 g/mol); however, significant decreases in both the T-g and E＇ resulted, indicating incomplete phase separation of the thermoplastic. A linear hydroxy-terminated polyester (M-n similar to 5400 g/mol) with a repeat unit structure which was similar to the HBP＇s was prepared and used as a control. The linear polyester (LPE) toughened the BMI nearly as effectively as did the HBP and caused a smaller decrease in the T-g and E＇. The viscosity of solutions of HBP and LPE in B were essentially the same at lower loadings in B, but at higher loadings, the HBP viscosity increased faster than did that of the LPE. The viscosity increase was end group-dependent. Preliminary morphological results are presented to show the effect of the thermoplastic architecture, loading, and end group on the cured thermoset.