How and when I do I reclass?

Because your MOS is currently under strength, you will have to wait until the next in and out calls message is released and your MOS becomes balance and then submit a voluntary reclass. That probably will not happen because Sergeant's are also under strength in your MOS. You would also have to apply before you pick up your promotable status because 68w does not accept promotable E4 and above.
There are two ways around this. The first is that you can request an exception to policy for voluntary reclass, you're 4187 will need to be signed by your Brigade Commander. Your new MOS will still need to be more understrength than the old MOS. Second way contact MOS proponent at the schoolhouse oh, not the branch manager, and ask for a grade waiver once you are promotable that will allow you to request the MOS once you become promotable and would otherwise be ineligible for it

If you want to be a medic without worrying about reclass in/outs for 68W head over to your SORB and talk to the SF or CA recruiter. Both have Special Ops Combat Medic/18D opportunities to become a medic but not worry about reup/in and outs.

Also, you will have needed to be on station for a year. If I remember correctly, you recently PCS to your new Duty station not that long ago