Conventional to name the shortest linkage $s$, the longest link $l$ and the other two links $p$ and $q$. These symbols also denote the length of the link.
There are then three unique assemblies of these linkages which are
Note that the order doesn't matter so $spql$ is identical to $pqls$, $qlsp$, $lspq$, $lqps$, $slqp$, $pslq$ and $pqsl$, that is rotating last letter to first, and reversing all combinations.
For the four bar linkage to be connected the minimal condition is that $spq<=l$
If one of the links do a full 360 degrees of rotation it is then considered as a crank. For the link $s$ to crank requires $s+l\le p+q$.
Links that don't crank are considered to be rockers.
See
It is often convenient to consider any one of the links as being a ground there after it is likely that a crank or rocker link will be driven by an external mechanism such as a motor so can be considered as the input or independent link. The link that is not connected to the ground link is known as the floater and the link of interest (possibly the floater) is considered as the output link. Note that often the output link may drive a further mechanism via a joint that can be at any point in its frame of reference.