Bataille Empire

Consider the situation below:

11
1122


3344
3344

Each character represents one base. 1 is infantry. 2 is a unit of detached skirmishers. Opposing them are two units of infantry 3 and 4

The two lines are 3 UD apart. Unit 4 is musket armed.

Unit 1 charges unit 3. May the detached skirmishers provide fire support to this combat or are they restricted to only firing at unit 4 as that is their normal target priority?

The fire support rules state "must be able to fire on the unit". I would suggest that the skirmishers would not normally be allowed to fire on the unit because they always have a higher priority target ahead of them, unit 4. Hence they cannot give support.
Unit 4 will be under fire so not able to give support either.
I would presume the intention is straight forward. A supporting unit cannot be engaged with other enemy.