'That was/is' can usually be followed by a plural-form noun phrase when the communication / event / group of people etc can be considered unary, for those allowing logical agreement.
- 'That was the 9-o'clock news and weather.'
- 'That was Mr and Mrs Johnson we were talking to.'
- 'That is/was //That's the Jehovah's Witnesses at the door.'
- 'That is/was //That's bacon and eggs as it should be cooked.'
.................
'That was the four ways ...' implies, as alphabet says, 'that program/demonstration showed the four ways'; 'those were the four ways' would emphasise the four ways themselves and is certainly correct. There are contexts where the former variant would not be reasonable:
- Indicating that something (especially something unexpected) is really true, a polite corrective marker, a marker politely added to a statement others might not have expected for you, and a polite topic reorientation marker – those are the four ways that you can use 'actually'. (see Collins)
And while the example
- That was his father and grandfather we saw in the cafe.
is the one I'd choose ('those were' sounds impersonal)
- That was /those were farmer Brown's sheep.
are both available, and I'd choose whichever I thought fitted the context. 'That was' highlights the perhaps unexpected meeting the flock, 'those were' is either unmarked or stresses the ownership (perhaps they'd strayed several miles).
With pluralia tantum, I'd rarely use 'That is ...', though possibly 'That's ...' (and then only with certain examples):
- Those are real Levi’s 501 jeans that Kim is wearing.
- Hey, those are my trousers!
- Those were the binoculars they recommended.
- That's the remains of a Joshua tree.
There are cases where 'that's' is used non-referentially (as in 'that's that'):
- That's the tidying up done, then.
- That's the summer holidays over for another year.
- That's the banns read.
Here, 'That's' is used no matter what appropriate noun phrase follows.