(Sorry for the slow answer – 3 days "hold" for new members here)
The Westphalian infantry in Catalonia were the 2nd, 3rd and 4th Line Regiments (2 batallions of 6 companies each) and the 1st Light Battalion (6 companies).
The last regular unit of Westphalian infantry to leave Spain was 1/4th Line, on 1 April 1811.
The remaining soldiers, some at that time in hosptial, were organized as the "bataillon léger westphalien" and continued to serve with the army of Catalonia, under the command of Oberstlieutenant Johann Theodor v. Winkel (chef de bataillon Jean-Théodore Winckel : Halberstadt 1767 – Netherlands 1827>), formerly commander of 2/4th Line. As light infantry, and a composite unit, the bataillon léger westphalien would have no flags or fanions.
They were at the re-taking of Figueres in 1811 with an état-major of 9 and 4 compagnies of chasseurs : 198 présents, 49 détachés, 171 aux hôpitals (total : 467)
Later they were posted as detachments to La Selva, Cadaqués and Roses.
On 15 October 1811 their strength was 20 officers and 215 other ranks present under arms.
On 6 December 1811, Oberstlieutenant v. Winkel was named a chevalier of the Légion d'honneur.
On 3 December 1812, King Jérôme requested the return of the light battalion in Spain, then numbering 21 officers and 283 other ranks and located at La Jonquera.
The battalion was first re-organized as two companies, and 6 officers and 27 sous-officiers were sent back, arriving in Kassel on 5 March 1813.
Then the remaining two companies, and a handful of artillerymen, were ordered home on 10 January 1813, arriving in Kassel on 14 April 1813.
On their arrival to Kassel, Oberstlieutenant v. Winkel's troops were incorporated in the newly created 4th Light Battalion under Geneva-born Oberstlieutenant Charles-Pierre Gauthier.
Although assigned to the 3rd Light Battalion, left behind tending the sick and wounded was chirurgien aide-major Monbert, who was mortally wounded on 7 February 1813 near Figueres.
Oberstlieutenant v. Winkel himself was posted as second in command of his original regiment, the 4th Line – but was actually put in command of the 2nd March Regiment (2 companies of fusiliers from each of the depots of the 5th, 6th, 7th and 8th Line, and a half-company of light infantrymen), taking replacements to the field army. He later entered the Dutch service, and was lieutenant-colonel commandant du dépot de la 8e division d'infanterie in 1817 and 1818, after which he retired with a Dutch pension.
The composite light battalion in Spain in 1811 and 1812 was not identified as the "4th".
French secondary sources (even very good ones) often mis-identify the 1st Light Battalion, which served at the storm of Gerona in 1809, as the "4th" – but this unit returned from Spain in early 1811. This 1st Light Battalion, under Oberstlieutenant Leopold Maximiliaan Julius v. Meyern (from 1815, Freiherr v. Meyern-Hohenberg), was the only Westphalian light batallion until the 2nd and 3rd were fielded in 1811, when it became officially the "1st".
The 4th Light Battalion did not exist until early 1813, fighting its first action at Lahn in August 1813.
Uniforms 1808-1809 – the remants of which would have been worn by the bataillon léger westphalien in 1811-1812 – you might do them as a small company from each of the 4 parent formations ….