Haha, this can get even more complex and you can think of it in term of EV (expected value). For this analysis, let's use a bet of 12 (pot of 84) just because you get nice whole numbers for two, three, and four-way draws), but it can be used for any bet size, of course. If you have a guaranteed two-way draw, you get 42

. If you three-way draw, you get 28

, and if you draw four-way, you get 21

. If you solo, you get all 84, of course.
Then it becomes a simple formula (assuming you are decent at estimating the chances of the various outcomes) to determine the EV of your stab and whether you "should" stab to maximize your value, or take the easy draw.
EV = 84a + 42b + 28c + 21

where A is your chance of the solo, B is your chance of a two-way draw, C is your chance of a three-way draw, and D is your chance of a four-way draw. My examples will only include two and three-way draws, for simplicity.
For example, if you have a decision to make whether or not to take your "guaranteed" two-way draw and finish off the third party or stab your ally and go for the solo, pretend you estimate you have a 60% chance of succeeding, 20% chance of a two-way draw, 10% chance of a three-way draw, and 10% chance of either being eliminated or leaving someone else to get the victory. You would have:
EV = .6(84) + .2(42) + .1(28) = 61.6
Because 61.6 is above your guaranteed gain of 42 for the sure two-way, you should clearly stab in this situation.
Using this analysis, the requirements of going for the solo may be lower than you expect. Assume you think you only have a 40% shot at the solo, and if you fail, you will most likely give another player the win, or be eliminated, 30% of the time, but have a chance to be included in a two or three-way draw at 15% each.
Gain = .4(84) + .15(42) + .15(28) = 44.1
In this case, trying for the stab is still the optimal decision, albeit only slightly. It just shows that even if your stab is more likely to fail than succeed, it can be the correct decision to try for it.
If you play poker, you can certainly understand this concept in terms of pot odds. When you are drawing for a flush, more times than not you are not going to hit, but it is a good decision to go for it if the odds are right.
I didn't expect this to get so technical but I think it can be an eye-opener to people who have never thought of it in these terms before.