Intrinsic disorder is at the center of biochemical regulation and is particularly overrepresented among the often multifunctional viral proteins. Replication and transcription of the respiratory syncytial virus (RSV) relies on a RNA polymerase complex with a phosphoprotein cofactor P as the structural scaffold, which consists of a four-helix bundle tetramerization domain flanked by two domains predicted to be intrinsically disordered. Since intrinsic disorder cannot be reduced to a defined atomic structure, we tackled the experimental dissection of the disorder-order transitions of P by a domain fragmentation approach. P remains as a tetramer at temperatures above 70 °C, but shows a pronounced reversible secondary structure transition between 10 and 60 °C. While the N-terminal module behaves as a random coil-like IDP independent of tetramerization, the isolated C-terminal module displays a cooperative and reversible metastable transition. When linked to the tetramerization domain, the C-terminal module becomes markedly more structured and stable, with strong ANS binding. Therefore, the tertiary structure in the C-terminal module is not compact, conferring "late" molten globule-like IDP properties, stabilized by interactions favored by tetramerization. The presence of a folded structure highly sensitive to temperature, reversibly and almost instantly formed and broken, suggests a temperature sensing activity. The marginal stability allows for exposure of protein binding sites, offering a thermodynamic and kinetic fine tuning in order-disorder transitions, essential for the assembly and function of the RSV RNA polymerase complex.