As Gecode 4.0.0 is a major revision, your programs that are written for Gecode 3.7.3 are likely to require some modifications before they work again.
The pre-defined variable-value branchings have been extended considerably and hence some changes are required.
In order to change, you have to add () to all variants of INT_VAR, INT_VAL, INT_ASSIGN, SET_VAR, SET_VAL, and SET_ASSIGN. For example, INT_VAR_MIN_MIN becomes the function call INT_VAR_MIN_MIN() and INT_VAL_MIN becomes the function call INT_VAL_MIN(). Some of these functions expect additional arguments and can take also optional arguments (this replaces the VarBranchOptions and ValBranchOptions). Please read MPG for details.
Variable selection for branching used the quotient of size divided by degree, accumulated failure count, or activity. Now the inverse is used. That is, for example, it is not any longer INT_VAR_SIZE_DEGREE_MIN() but INT_VAR_DEGREE_SIZE_MAX() (that is, largest degree divided by size).
That looks like an annoying change but is in fact essential: the strategies using accumulated failure count and activity now could have run into division by zero issues. And just changing the implementation is not good enough because the values of these measures can now be exposed during tie-breaking.