Does it matter if you have three or four possible answers when making a multiple choice test for learning?
The standard for multiple choice questions is normally 4 answers. However, there is research that indicates that three choices is optimal. Three choices seems to increase the reliability of the test and decreases the number of poor distractors. 

If you are worried about the difficult of your assessment. Instead of developing questions with four choices another strategy would be to increase the number of questions with three choices. This will allow you to cover more content.