Verbs can precede subjects more or less whenever you want them to. It typically looks something like this:
The dog jumped over the fence.
Over the fence, the dog jumped.
As RegDwight notes in the comments, this is called a hyperbaton:
Hyperbaton is a figure of speech that consists of an alteration of the logical order of the words in a sentence, or in which normally associated words are separated. The term may also be used more generally for all different figures of speech which transpose natural word order in sentences.
A whole bunch of detail and examples can be found on this previous answer to Why is this a hyperbaton?
For your particular example, there is no comma:
In fig. 4 is shown [...].
The more standard form is actually:
[...] is shown in fig. 4.
It is common in technical writing to move "fig. 4" to the beginning of the sentence because it is easier to read along with the description once you know which figure to look at. Why they bothered keeping "in" and "is" is beyond me. It would be much easier to read without them:
Fig. 4 shows [...].