On marksdwarves: notoriously difficult to get working effectively, for all sorts of reasons. However, this is how I do and it's been suiting me fine for 10 in-game years:
One large archery range with 10 targets, each of which have a range designated from them, all sorted direction-of-fire-wise, all set for the single squad to train from. Plus, one barracks from an armour stand, also designated for that one squad to train at. Schedule: one month training, one month off, alternating.
In the training months, the squad normally goes and does dodging practice in the barracks, sometimes with one or two or them going and firing a few arrows. As soon as their off-duty month arrives, about 8-9 of them go shoot, and since they have no other labours enabled while off duty, they do that for the entire month.
I do a bit of live target practice (trolls, bugbats, elk birds, anything else I catch) through fortifications.
Note: when I say off-duty, I am not setting the squad to 'inactive' ('t' on the 's'quad menu), simply setting their schedule to have 'off' months. Just to clarify.
Been working fine. More generally speaking though, marksdwarves do have a lot of things to juggle: bolts, quivers, kit, flasks, backpacks, etc, so do just give them time. Your idea of stationing them was a good one - why not wait until all four squads members are present and correct and you can check their kit and then un-station them and give them a bit of time to get themselves into some kind of training routine.