Bridge players use a more fine-grained point system, where 3 points is worth roughly a trick. The short-suit evaluation used then is something like the following:
Plus a 5th trump is evaluated as 1 point, and each additional trump beyond 5 as 2 points.
Part of the reason why these evaluations may seem a bit lower than expected is that there may be duplication with partner. In Bridge such duplication is assumed, and the hand perforce loses value if it doesn't exist.
From my comment below:
[This evaluation] has it's origins in the Work Point Count popularized first by Milton Work and then Charles Goren. There are variations, as no definitive research exists, but all are similar to what I outline above.
Other evaluation systems exist, including Larry Cohen's Law of Total Tricks and Courtenay's Losing Trick Count, which experts use as a refinement and/or double-check on the Point Count evaluation. Point Count tends to give a best estimate of strength while the other two tend to give a maximum estimate of hand strength.
Culbertson's Honour Tricks, now usually referred to as Quick Tricks, are an older assessment of hand strength now usually used as an adjunct to Point Count. Thus various actions in a Contract Bridge auction will have a minimum (occasionally also a maximum) in both Point Count and Quick Tricks.