Prim alignment and scaling accuracy

I've been building all kinds of objects with quite a precise level of detail, and I've noticed that you can place prims much more accurately if you move it into place by dragging the XYZ-handles rather than entering values in the edit window. Easiest way to prove this is to create any object the size of 0.01x0.01x0.01, zoom in as close as possibe, drag the handles a little and you can see that the coordinates of the object do not change, although the position does.

Also, the scale of an object can be changed much less than 0.001 meters by linking the object with another object and stretching them both simultaneously, then unlinking them again.

This is a tip.

The question is, why isn't a fourth decimal shown in the editor?

I don't know why they don't. Maybe there's a messiness factor in having so many digits, but that's just a guess. It's also hard to observe exactly to what level of detail the position can be edited, as prims have a way of temporarily appearing set to any values you assign them, before snapping back into the closest value that SL is able to store.

What I do know is that some values, and not just the position, can be edited to a higher level of detail than the display suggests. For example, I have found that I am able to set Hollow to .01 (which has enabled me to make little spheres 1μm (a micron) across...), by just typing that number in.

So always try adding extra digits to your numbers, even if you can't see those digits displayed back to you.

I'm pretty sure that the numbers are internally just standard computer floating-point numbers, which means they've got a pretty high accuracy. They are set that way in scripts, at any rate; I think the limit in the interface is just exactly that they wanted to keep the interface reasonably clean and someone made a judgement call about how many digits was enough. They could have had it show eight digits just as easily.