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The call glDrawElements(..., indices.size(), GL_UNSIGNED_SHORT, 0); tells your GPU roughly following:

for(unsigned short i = 0 /*last parameter*/; i < indices.size(); i += 3 /*3 vertices/triangle*/ )
{
   drawTriangle(VBO[indices[i]], VBO[indices[i + 1]], VBO[indices[i + 2]] );
}

Do you see what is wrong now? If the value of indices[i] is higher than your size of VBO, it is a problem.
Moreover, for VBO it is recommended to have around 1-4MB of vertices in it - only 4 vertices/VBO will very likely kill your performance.

big edit reason: cant read right(solved in comments)

The call glDrawElements(..., indices.size(), GL_UNSIGNED_SHORT, 0); tells your GPU roughly following:

for(unsigned short i = 0 /*last parameter*/; i < indices.size(); i += 3 /*3 vertices/triangle*/ )
{
   drawTriangle(VBO[indices[i]], VBO[indices[i + 1]], VBO[indices[i + 2]] );
}

Do you see what is wrong now? If the value of indices[i] is higher than your size of VBO, it is a problem.
Moreover, for VBO it is recommended to have around 1-4MB of vertices in it - only 4 vertices/VBO will very likely kill your performance. You might be interested in instancing instead.

big edit reason: cant read right(solved in comments)

The call glDrawElements(..., indices.size(), GL_UNSIGNED_SHORT, 0); tells your GPU roughly following:

for(unsigned short i = 0 /*last parameter*/; i < indices.size(); i += 3 /*3 vertices/triangle*/ )
{
   drawTriangle(VBO[indices[i]], VBO[indices[i + 1]], VBO[indices[i + 2]] );
}

Do you see what is wrong now? You need index for each vertex drawn. In you draw method you told GPU to render indices.size()(which happends to be 6) vertices. Additionally, there might be one more error in specifying size/leght in some other glCall(I would expect last 2 triangles drawn).
A somebody gave me a good advice: when everything fails, start drawing. I dont say this is copy/paste solution, but hopefully it gives you an idea how to solve similar problems, taking pencil and paper and getting right indices manually gives: ![enter image description here][1]

...it should not be difficult to derive right formula (you probably already have the right one, just you need to apply if for every "index" = for every quad and handle looping from back of quad "strip") [1]: https://i.sstatic.net/n9QZY.png

The call glDrawElements(..., indices.size(), GL_UNSIGNED_SHORT, 0); tells your GPU roughly following:

for(unsigned short i = 0 /*last parameter*/; i < indices.size(); i += 3 /*3 vertices/triangle*/ )
{
   drawTriangle(VBO[indices[i]], VBO[indices[i + 1]], VBO[indices[i + 2]] );
}

Do you see what is wrong now? If the value of indices[i] is higher than your size of VBO, it is a problem.
Moreover, for VBO it is recommended to have around 1-4MB of vertices in it - only 4 vertices/VBO will very likely kill your performance.

big edit reason: cant read right(solved in comments)

The call glDrawElements(..., indices.size(), GL_UNSIGNED_SHORT, 0); tells your GPU roughly following:

for(unsigned short i = 0 /*last parameter*/; i < indices.size(); i += 3 /*3 vertices/triangle*/ )
{
   drawTriangle(VBO[indices[i]], VBO[indices[i + 1]], VBO[indices[i + 2]] );
}

Do you see what is wrong now? You need index for each vertex drawn. In you draw method you told GPU to render indices.size()(which happends to be 6) vertices. Additionally, there might be one more error in specifying size/leght in some other glCall(I would expect last 2 triangles drawn).
A somebody gave me a good advice: when everything fails, start drawing. I dont say this is copy/paste solution, but hopefully it gives you an idea how to solve similar problems, taking pencil and paper and getting right indices manually gives: ![enter image description here][1]

...it should not be difficult to derive right formula (you probably already have the right one, just you need to do if for every "index" inside loop creating quad vertices) [1]: https://i.sstatic.net/n9QZY.png

The call glDrawElements(..., indices.size(), GL_UNSIGNED_SHORT, 0); tells your GPU roughly following:

for(unsigned short i = 0 /*last parameter*/; i < indices.size(); i += 3 /*3 vertices/triangle*/ )
{
   drawTriangle(VBO[indices[i]], VBO[indices[i + 1]], VBO[indices[i + 2]] );
}

Do you see what is wrong now? You need index for each vertex drawn. In you draw method you told GPU to render indices.size()(which happends to be 6) vertices. Additionally, there might be one more error in specifying size/leght in some other glCall(I would expect last 2 triangles drawn).
A somebody gave me a good advice: when everything fails, start drawing. I dont say this is copy/paste solution, but hopefully it gives you an idea how to solve similar problems, taking pencil and paper and getting right indices manually gives: ![enter image description here][1]

...it should not be difficult to derive right formula (you probably already have the right one, just you need to apply if for every "index" = for every quad and handle looping from back of quad "strip") [1]: https://i.sstatic.net/n9QZY.png