Tiers of scaffolds are the structure of a central leader tree.

J. plums are usually trained to an open center even if started as a central leader and often I start with as many as 5 scaffolds or more, depending on the trees ability to produce secondary branching quickly. Scaffolds can be thinned as crowding makes necessary.

I don't know how J. plums behave on dwarfing rootstock but certainly as long as branches aren't more than half the diameter of the trunk at point of attachment it'll be fine, but avoid branches with inverted bark.

4 ft between tiers is a scale for trees that are to eventually be between 10 and 16 ft. For full dwarf central leader trees I don't even think in terms of tiers.

If anyone else has thoughts on this please chip in, I like a discussion and I'm curious how others train their J. plums. I've always wondered how repeated heading cuts might work in developing secondary branching but I can't bring myself to experiment with it. I just head back long lanky branches to a secondary- even if it's smaller than I'd like and the trees eventually do what I want them to as far as filling out.

This link shows like 4 scaffolds all on the original tier. Are you suggesting a second tier of 4 scaffolds?

http://www.ces.ncsu.edu/depts/hort/hil/ag29.html

On free standing apples trees that produce abundant secondary wood I prefer 3 branches on bottom tier and 4 on the tier above and that's all the scaffolds the trees I manage need when trained to the Christmas tree form. My first tier starts higher than they would in a commercial orchard.

How can I spot this?

"avoid branches with inverted bark."

Inverted bark is created by pressure when the scaffold branch grows as fast as the trunk. When you see it it becomes obvious and occurs with narrow crotches in particular on the upper side of the branches union with the trunk and consists of overlapping tissue. I'm sure if you google inverted bark, tree pruning you can find a picture.