I've been running on the original external bb since I bought my bike 4 years ago. When I disconnect my chain, lean it on the frame and spin the cranks freely, there seems to be a small point in the turn where there is some slight resistance. Is it time to service this thing? If so. How exactly is this done? Do I have to buy a whole new external bb or can I replace the cups on the externals that I have now?

Some slight resistance in my opinion is nothing to worry about, but the bearings can be replaced. Google for the procedure.

Rule No.1 ---- if it ain't broke, don't fix it. From your description the bearings are fine. Unfortunately (or not) you can't open them up for an inspection, so have to rely on the fact that they still turn relatively smoothly, without excess play or roughness to indicate wear. So enjoy your good fortune while it lasts.

If/when it's time to replace, you'll most likely need to replace the 2 halves of the external BB. Most of these have pressed in bearings, and don't provide for easy replacement of the bearing itself, though it's probably doable. Even so, I suspect that you'll find the cost of the bearings alone to be close to the complete BB.

One caveat for future reference. The most serious issues with this type of BB are the cups seizing in the frame because of corrosion, and a seized bearing. The seized bearing is critical because the spindle can still turn within the inner race, and riding that way will score the spindle setting up a future fracture. If you feel or hear friction, check what's happening immediately, by sliding out the cranks, and seeing that the inner race still spins smoothly.

Okay so the slight resistance (which is mainly just a low-key fluffy noise) is negligible.

Checking it again, I also noticed that there is a slight play. If I hold the cranks on each side and try to wobble it, there is a small wobble and clicking noise with each wobble. Maybe 0.5-1mm wobble? Is this also negligible? Its not something that I notice or hear when I ride at any intensity be it standing or while sitting on the saddle.

So, it's showing wear, but still workable. Since nothing you do will make any difference, ride it as long as it works and replace if/when convenient. In situations like this, I buy the replacement so it's on hand, but keep using the original until I can't.

If this is a Shimano crank and BB Hollowtech I would just replace the BB. They are ridiculously cheap about $25 and that is whole point of why I like thread BB and external bearings, they are breeze to change and/or replace.