Lines Through Dots

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First, a classic:

1. Without lifting your pencil, draw four straight lines through 9 dots arranged in a 3x3 grid.

Now the extensions. Without lifting your pencil, and trying to begin and end at the same location,

2. Draw six straight lines through 16 dots in a 4x4 grid.
3. Draw eight straight lines through 25 dots in a 5x5 grid.
4. Draw ten straight lines through 36 dots in a 6x6 grid.

Source: Many puzzle sources.  Can you do it with fewer lines?