Well, it is important to be gentle and positive about how you
perceive your child or else EL can turn into a stressful thing that is 100% against what EL is all about. EL is about joyful learning, taking advantage of this time when it is effortless to learn and making the child view learning as an enjoyable, daily activity that brings them closer to their care giver.
It can be unhelpful for your to interpret your sons behavior are "not concentrating" because you are translating him by an adult lense but he is, in fact, an infant. Your goal is to gently engage him and be consistent. The first rule of EL is "Thou baby shalt have fun" and "Neither thou nor thou baby may be less than willing." If you think that your son isn't interested/annoyed the S-T-O-P right away and re-evaluate.
Here are some general tips.
1) Set up the lessons while your son is occupied doing something else. Do NOT make him sit through turning the computer on or setting up the days lesson.
2) Do one lesson and only one lesson at a time. Don't go through a lesson of all 3 at the same time if you think that your child isn't enjoying it.
3) Make sure baby is comfortable before beginning each lesson (clean, dry, well fed)
4) Sandwich the lessons between 2 things that the baby enjoys. (i.e. a song, lesson, game)
5) Play with your baby immediately after each lesson, so that he is eager to get through the work because he knows that mommy or daddy will blow his tummy right after the lesson.
6) Work your way up to adding in more lessons gradually and slowly.
7) Perhaps try plugging computer to display on TV so that words are bigger.

Try physical flashcards
9) Read books together.
Don't worry about your 4mo concentrating, worry about having positive experiences.