Use any drawing software like Paint or the like to draw 4 triangles using only 4 straight lines! Also, the borders of your drawing don't count ;)
I think you can draw
any four straight lines.
(As long as no two of them are parallel with each other, and all the six crossing points are distinct from each other.)
This works, because
any three straight lines will always form a triangle, unless either
* some two of the lines don't ever cross (because they are parallel), or
* all three lines cross at the same point.
Since you have 4 lines, you can
choose three of them in 4 separate ways,
so you are guaranteed to always get exactly 4 triangles.
Here's a solution for 4 triangles with 4 straight lines:
Here are the first two triangles:
Here's the third triangle:
Here's the fourth triangle:
I suspect that this will work for any triangle formed of three lines with an additional line, so long as the additional line is not parallel to any of the three other lines nor incident to the vertices of the triangle.