Question about the Karnaugh maps and the number of solutions
There are a lot of expressions in which a digital logical function can be represented. But the two canonical forms of any Boolean function are:

1. Sum of minterms: This will express the function as a OR (sum) of minterms. Hence called as sum of products (SOP) form. In your case it will be:

$$F = w'x'y'z' + w'x'yz' + w'xy'z' + w'xy'z + wx'yz' + wxy'z' + wxyz$$

There will be a term corresponding to each minterm (0,2,4,5,10,12,15). 

2. Product of maxterms: This will express the function as a AND (product) of maxterms. Hence called as product of sums (POS) form. In your case:

$$F = (w'+x'+y'+z)(w'+x'+y+z)(w'+x+y+z')(w'+x+y+z)(w+x'+y'+z')(w+x'+y'+z)(w+x'+y+z)(w+x+y'+z)(w+x+y+z')$$

There will be a term corresponding to each maxterm (1,3,6,7,8,9,11,13,14).

Non-canonical form:
It is often the case that the canonical minterm form can be simplified to an equivalent SOP/POS form. This simplified form would still consist of a sum of product terms/product of sum terms. 

And what you got here is the simplified SOP version of it. Similarly you can get the POS form of it.

Source:Wikipedia