Actually, I'd recommend NOT asking the toy store because if you say, "v4.5", they're going to look at you in a funny way. Haven't actually tried this but it's very very possible that they won't know what you're talking about.
The best places to check in Australia would be either Toys-R-Us or Target. Big W gets electronic stuff a lot later than anything else. I wouldn't try Games World either, in fact my nearby Games World doesn't even stock Tamagotchi anymore.