3 or 4 games seems a bit too little to get tired from. Does this happen with any other games they play, aside from Smash 4?
The brain uses 25% of a human body's energy. When playing to win, it's going to use a lot more. In fighting games, players are always in the action and always have to be responding and consciously thinking about what will happen. They have to observe their opponents, interpret patterns, and calculate/predict what could happen next, do certain actions that (what they think) are likely to work, and keep repeating that mental process repetitively until the game ends. Smash doesn't have the luxury, in terms of mental energy spending, of being able to relax a bit when getting comboed or comboing. Combos are very inconsistent and everything has to be reacted to
If your friends get bored easily, maybe the game or competitive play doesn't fit their tastes? If they know you're holding back, they might feel a bit disrespected to not be taken seriously