Use what is available in ANSI B 16.9 table 12. in your case it is 8 x 4, 4 x 2. even 8 x 6 would need a 6 x 3, 3 x 2.
The ycan look quite odd, especially if you've got eccentric reducecers, but it's allowable.
For really big changes (say 16 x 2) you could use tees, but it is fairly rare to go for that sort of size change.
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