A systematic study of the effects of charge composition and process parameters is presented for a prototype automotive part (a door cassette) produced by compression moulding from glass fibre and polypropylene composites. This follows previous studies (Parts I–III) of the process-property relations for commingled glass and polypropylene fabrics, glass-mat thermoplastics (GMTs) and sandwiched structures of commingled fabric shins with a GMT core. The scope and function of the component is described together with details of the moulding experiments with GMT and fabric combined as a pre-laminated sandwich and a GMT charge with local areas of fabric to provide selective stiffening. Process parameters were varied to study the effects on proportional mould fill, pressure distributions, microstructure and flexural modulus. Statistical process models were generated from the results and these were used to define process windows. Of the parameters studied, the consolidation time dominated the flexural modulus and void content whilst increased press force and material temperature increased proportional mould fill.