  We present an exact method based on a hyperspherical harmonic expansion to
study systems made of quarks and antiquarks of the same flavor. Our formalism
reproduces and improves the results obtained with variational approaches. This
analysis shows that identical-flavor four-quark systems with non-exotic
$2^{++}$ quantum numbers may be bound independently of the quark mass. $0^{+-}$
and $1^{+-}$ states become attractive only for larger quarks masses.
