After reading about lineup creation, player positions, and basics of formations, you should have a solid grounding to select your formation for the season. Here are specific recommendations for 4v4.
1-2-1
For 7u and above I recommend a basic diamond formation. It introduces the concept of a 3 tier system and promotes better spacing among players. The ideal spacing is roughly 10 steps apart so that your fullback and forward are always close to the action.
1-3
For 6u and below you may choose to have a single fullback and have the rest of the team “float”. Or you can try to assign a left, right, and center midfielder. It will be more about just introducing them to the topic of positions since all your mids will clump around the ball. If you can get the players playing fullback to hang back a few steps that will be a win.
3-1 is another option. You have a lone attacker and three defenders, a left, center and right. The left and right defenders are really like midfielders, they may travel up and down the full length of the pitch. However, when you tell them they are defenders, they start to get the idea of needing to get behind the ball when the other team has possession. It’s much easier to later tell a young player to push up than it is to push back.
Agree that 1-2-1 “diamond shape” works well and gives them a good basis for the formations used at higher divisions.