I have a 91 with 170k on it; I just rebuilt the head for similar reasons. Since your miata has so few miles your seals should be your only problem. The bad news is that the rest of your seals are pushing 14 years and may have dry rotted as well. Seals to look out for are: cam angle sensor o-ring, cam-seals, and crank-seal. Before you attempt to change your crank seal you should read this, it only applies to 90-91 miata's.
http://www.miata.net/garage/crankshaft.html
The other possibility is that you have put on an after market exhaust and did not properly torque your manifold bolts. I found this condition on my miata when I pulled the head off. My cylinder 3 and 4 (the ones you describe) had burnt valves. The bolts on those cylinders are harder to get at, and I must have been feeling lazy at that point in the day or thought I got them down tight. If this is the case the extra heat would harden the stem seals in those cylinders prematurely and cause the oil leak. But worse than that, it burnt my valves on those two cylinders. I performed leak down tests before and after the head rebuild and my compression went from 80-90psi to 170psi on my 3 and 4 cylinders. I would suggest you have a leak down test preformed on you engine. If your valves are burnt, you will hear air escaping from the intake manifold and possibly the exhaust when you perform a leak down test. If you don't have access to a leak down test, pull of you exhaust and look at the gasket, if you see black or carbon stains on gasket around the port leading out to the edge of the manifold, your bolts where loose and you had an slight exhaust leak.
Great info guys!
