There is method Enumerable#reject which serves just the purpose:
(1..4).reject{|x| x == 3}.collect{|x| x + 1}
The practice of directly using an output of one method as an input of another is called method chaining and is very common in Ruby.
BTW, map (or collect) is used for direct mapping of input enumerable to the output one. If you need to output different number of elements, chances are that you need another method of Enumerable.
Edit: If you are bothered by the fact that some of the elements are iterated twice, you can use less elegant solution based on inject (or its similar method named each_with_object):
(1..4).each_with_object([]){|x,a| a << x + 1 unless x == 3}