The regiment was apparently present at Fuentes De Onoro under Barbacena with (according to Oman) a mere 104 men, and weren't much engaged after that:
The 4th however represents another of the cavalry regiments from the Central Division, so it is possible that what few mounts they had may have been absorbed into other regiments.
The white piping is indicative of the Central Division, in this case with a red collar:
I have again utilised the Front Rank "at rest" figures for them, which I quite like even though the pose is perhaps a little less dynamic than some others:
That's two cavalry regiments completed now, with just the 11th Almeida to go. Hopefully I'll have them completed in the next week or so.