If you want to learn more, check out the Introduction to Bambu Nozzles Wiki article:
Is there any way of measuring the nozzle hole?
You can, but only with more advanced tools.
At home, it’s difficult to measure it accurately.
Another way to determine whether the nozzle may need replacing, in addition to the methods described above, is to compare it visually with a new nozzle side by side.
If you can see a noticeable difference between the new and used nozzles, it may be time to replace the nozzle.
Using only non-abrasive (PLA and PETG) filament, what is the average useful life of a nozzle in hours?
Microscope and compare the hole size to a new nozzle.
Perhaps if you extrude a straight line in air, then take a brand new nozzle and extrude the same line with the same settings, you can measure the two lines and compare them to get some indication of how much wear there is, at least in the orifice diameter anyway.
Do you even have to dry pla?
Would it hurt? Any filament is exposed to a great deal of moisture in manufacture (usually cooled in a water bath after extrusion) and even tho PLA is not wildly hygroscopic, it isn’t a bad idea to dry it at least once when freshly opened. After that you may not have to go to the extremes to keep it dry as you do with things like PETG and TPU. I keep PLA in bags or boxes as much to keep it clean (dusty house) but I do always throw some desiccant in with it. Can’t hurt.
I have had my HF.4 nozzle for around 6 months. Should I replace it?