¹²CO(4–3)/¹²CO(2–1) integrated brightness ratio (black) for J-type shock models from the Paris-Durham code. The ratio at a shock velocity of 4 km s⁻¹ is higher than observed in Musca. This is expected because the ratio decreases towards lower velocities and the ¹²CO(2–1) emission in Musca also comes from non shocked gas, such that the observed ratio is a lower limit for the shock models. In red, we show the integrated brightness of ¹²CO(4–3) for the same shock models, together with the ¹²CO(4–3) integrated brightness interval of the blueshifted component in Musca. The observed values in Musca show a relatively good agreement with the predicted ¹²CO(4–3) integrated brightness for a pre-shock density of ~ 5 × 10² cm⁻³.