See How do I relax the integrality of only some of a model’s integer variables? in our Frequently asked questions. Also, there is an example in a recent post about Relaxing integrality on specific index values of a variable.
For your example, you could define
var x {i in 1..n} integer >= 0;
and then use these statements to say that x[4] must be an integer that is >=5:
let {i in 1..n} x[i].relax := (if i = 4 then 0 else 1);
subject to LowerBound: x[4] >= 5;