Same as they always were in the other versions
10.30am, 3pm and 7pm (by your tama's little clock)..
The main difference with the V4 is that:
1) the Matchmaker doesn't stay on screen for very long at all - you still get the attention beep to let you know she's arrived, but she only stays for 10 seconds - so be ready for her!
2) sometimes you get a mail delivery at these times so the Matchmaker doesn't show up and you miss your chance to get a partner
If you miss her at one of the times and you can't wait til the next visit, just do what lots of us do - change your tamas clock to 10.29am, 2.59pm or 6.59pm - wait one minute and be ready to choose a partner
If you don't find a mate with the Matchmaker or by connecting with other tamas, your tama will evolve into an Old Character (Otokitchi - Female / Ojitchi - Male).