“O mistress mine, where are you roaming? / O, stay and hear; your true love's coming,
That can sing both high and low: / Trip no further, pretty sweeting;
Journeys end in lovers meeting, / Every wise man's son doth know.”
I think the lines that you are asking about are the above lines. At this point in the play, all sorts of mix ups have happened. They will continue to get more convoluted as the play continues. A love triangle between Viola, Orsino, and Olivia is just the beginning of all of the mix-ups in the play. Viola believes her brother to be dead, Malvolio is just a ball of negativity, etc. Then Feste sings the above lyrics to Sir Toby and Sir Andrew. It speaks of lovers finding each other, and a love journey coming to a happy ending—which is exactly what happens at the play's conclusion when Viola and her brother are reunited and the love triangle is resolved as the couples wed.
In scene 4, Feste's song is about unrequited love, which Orsino, Viola, and Olivia are all experiencing. The song is about how that man dies of sorrow. It's a form of matchmaking, because it's inadvertently encouraging the listeners to not worry about unrequited love. Forget about it. Go find someone else, or you will die sad and lonely. It's very much in line with what Benvolio tells Romeo in Act 1 Scene 1 of Romeo and Juliet: forget Rosaline; there are plenty of other women. Feste's song is pushing his listeners to do likewise.