I see now Garmin Pay support Japanese IC Card Suica. If I already have vivoactive 4 bought outside Japan and travel to Japan , can I add suica card to my vivoactive 4 and use for pay train transit or not?
The Garmin Japan site says it is only available for Japanese versions.
Japan's myriad NFC services like Suica, iD, Nanaco, Edy, etc. all use the Sony-developed FeliCa (NFC-F) standard, which is different from most services in the rest of the world which use NFC-A/B.
As a result, a lot of manufacturers leave out NFC-F support to save money unless a device is specifically targeted at the Japanese market. Many people visiting Japan are bummed out to find that they can't put Suica or other Japanese NFC services on their foreign phone because it doesn't support FeliCa. (Unless they are iPhone users because Apple doesn't make a specific Japanese version of its phones, so all of its phones support FeliCa)
I don't know if this is the reason why Garmin Pay isn't available on non-Japanese watches, but it's likely to be the case. Otherwise it might be a licensing issue with JR East.
If you are only visiting Japan temporarily, virtual Suica cards like the one offered with Garmin Pay aren't a good deal anyway because you cannot get the balance refunded when you no longer need it unless you have a Japanese bank account for them to transfer the funds to. It's better to get the card version because you can get the balance refunded as cash when you leave. Or you can just spend the balance and keep the card as a memento.