What are the names of these forms?
1)could + infinitive (es they could go)
2)could have + past participle of verb (ex. they could have worked)
3)could have been (they could have been working)
4)could be (they could be working)
They are all modal forms because 'could' is a modal verb. "Modals" is the general name. Various books may have different names for the forms you cited. I am not aware of any universal standard set of terms for naming them.

I would use the following terms.

could/may/might/can/should/ ...

go modal
be going modal progressive
have gone modal perfect
have been going modal perfect progressive

Other names are:

could/may/might/can/should/ ...

go modal with infinitive
be going modal with progressive infinitive
have gone modal with perfect infinitive
have been going modal with perfect progressive infinitive

As you see, I prefer to use the less wordy descriptors.

The term 'continuous' substitutes for 'progressive' in some textbooks.