  We propose to measure the yields of $^4{\rm He}$ and $^4{\rm Li}$ in
relativistic heavy-ion collisions to clarify a mechanism of light nuclei
production. Since the masses of $^4{\rm He}$ and $^4{\rm Li}$ are almost equal,
the yield of $^4{\rm Li}$ predicted by the thermal model is 5 times bigger than
that of $^4{\rm He}$ which reflects the different numbers of internal degrees
of freedom of the two nuclides. Their internal structure is, however, very
different: the alpha particle is well bound and compact while $^4{\rm Li}$ is
weakly bound and loose. Within the coalescence model the ratio of yields of
$^4{\rm Li}$ to $^4{\rm He}$ is shown to be significantly smaller than that in
the thermal model and the ratio decreases fast from central to peripheral
collisions of relativistic heavy-ion collisions because the coalescence rate
strongly depends on the nucleon source radius. Since the nuclide $^4{\rm Li}$
is unstable and it decays into $^3{\rm He}$ and $p$ after roughly $30~{\rm
fm}/c$, the yield of $^4{\rm Li}$ can be experimentally obtained through a
measurement of the $^3{\rm He}\!-\!p$ correlation function.
