Let's give partner a fitting minimum for the first auction; one that fits nicely but is otherwise as weak as possible:
This hand by partner has only 11 HCP, evaluates to exactly 13 total points as opener, and there are excellent chances to make 4S, especially with an attacking diamond lead being up to partner's holding.
Let's also consider a non-fitting maximum hand for partner, with the maximum possible non-working values:
Again 4S has a good play, with one sure loser in trumps and good chances that the rest of the hand can be managed for only two losers.
These two hands illustrate the principle that if one's own values are strongly fitting, as in the hand given, and especially when one is the weaker hand of the partnership, one can be aggressive. Practice visualizing typical fitting minimum and non-fitting maximum hands for partner in the bidding, and evaluate your own holding against those when deciding whether to bid or invite game/slam. Caveat: Be very careful to not place cards in partner's hand when doing so. It is easy to engage in wishful thinking when doing this exercise, so you must consciously restrain your self until that becomes second nature.
Let's try the flip side; partner's fitting minimum could be something like:
This is about as bad a hand as partner could come down with, 8HCP and 10 total points, yet 4S is not hopeless despite the mismatch in the red suits. Even small additions like ST or C9 start looking like big cards. Vulnerable at IMPS bidding this game is mandatory, and perhaps also non-vulnerable at IMPS, but at MatchPoints I'd rather play 3S and trust partner to bid game herself when holding a hand like
The three distinctive marks of a weak player are:
Note that the examples of hand visualization given above all start with counting - of points, distribution, potential losers and winners - of things that cannot yet be seen in partner's hand but must be visualized. One begins to approach expert status only when this process starts to become second nature. In many senses an expert performs this visualization first; decides on the appropriate call based on this visualization; determines what the value of the hand is based on the call and bidding system; and only then decides how that value is apportioned as HCP and distribution and fitting points.