.container {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(300px, 1fr));
    grid-template-rows: repeat(auto-fit, minmax(300px, 1fr));
}

=============
= 1 = 2 = 3 =
= 4 =   =   =
=============

=============
= 1 = 2 = 3 =
= 4 = 4 = 4 =
=============