A parallelogram.
The regular tetrahedron - one of the 5 platonic solids.
A four sided figure would be a tetrahedron and a 20 sided would be an icosahedron. They both have equilateral triangles for faces so your question is a little incomplete.
tetrahedron
Four equilateral triangles make up another equilateral triangle.
A tetrahedron is a three-dimensional figure with four equilateral triangles. Every pyramid with a triangle as the base is a tetrahedron.
A tetrahedron has four equilateral triangles as sides
square
Rohmbus is a four sided figure (quadrilateral) that is made up of two equilateral triangles
A three-dimensional figure or shape, such as a tetrahedron, has four faces. These faces are equilateral triangles. A tetrahedron also has four vertices and 6 edges.
The regular tetrahedron - one of the 5 platonic solids.
A four faced figure is a tetrahedron. If they are all congruent triangles, they are all equilateral triangles and it is a regular tetrahedron - one of the 5 Platonic Solids.
Cross two match sticks to bisect like X and place the other two match sticks at the base of the two equilateral triangles formed .
A four sided figure would be a tetrahedron and a 20 sided would be an icosahedron. They both have equilateral triangles for faces so your question is a little incomplete.
tetrahedron
Put four equilateral triangles so that each one of them has a vertex at a single point and the triangles abut one another. The shape will be 4/6 (= 2/3) of a regular hexagon.
Put the triangles in a line alternately pointing up and down.
The tetrahedron. If one of the four triangles is equilateral and all others are isosceles, then one might also call it a (triangular-based) pyramid.