I computed those numbers via exhaustive search through all possible combinations. Doorcards are assumed to be neutral/unknown. Maybe I'll do some computations with known doorcards if someone comes up with an interesting example. Feel free to verify the numbers by comparing with charts from the net or via manual calculation.

3rd Street:

We start with a quality hand, 3 suited cards, such as



Probability of making an 8-or-better-low by showdown: 48%
Probability of making a flush by showdown: 18% (PFO tournament: 99%)
Probability of scooping by showdown: 10% (assuming qualifying low and flush are best)

4th Street:

We catch a nice card:



Probability of making an 8-or-better-low by showdown: 71%
Probability of making a flush by showdown: 47%
Probability of scooping by showdown: 35% (assuming qualifying low and flush are best)

Note: A23 suited and similar hands are underdogs against trips. After improving on 4th as shown above, they're favorite, and you should start betting like a maniac. There's virtually no hands you're a substantial underdog against now. Of course in a multi-way pot you might want to keep in other players with weaker hands and just call bets.