  We study the populations of stellar clumps in three high-redshift galaxies,
at z=4.92, 4.88 and 4.03, gravitationally lensed by the foreground galaxy
clusters MS1358, RCS0224 and MACS0940, respectively. The lensed galaxies
consist of multiple counter-images with large magnifications, mostly above $\rm
\mu>5$ and in some cases reaching $\rm \mu>20$. We use rest-frame UV
observations from the HST to extract and analyse their clump populations,
counting 10, 3 and 11 unique sources, respectively. Most of the clumps have
derived effective radii in the range $\rm R_{eff}=10-100$ pc, with the smallest
one down to 6 pc, i.e. consistent with the sizes of individual stellar
clusters. Their UV magnitudes correspond to $\rm SFR_{UV}$ mostly in the range
$\rm 0.1-1\ M_\odot yr^{-1}$; the most extreme ones, reaching $\rm SFR_{UV}=5\
M_\odot yr^{-1}$ are among the UV-brightest compact ($\rm R_{eff}<100$ pc)
star-forming regions observed at any redshift. Clump masses span a broad range,
from $10^6$ to $\rm 10^9\ M_\odot$; stellar mass surface densities are
comparable, and in many cases larger, than the ones of local stellar clusters,
while being typically 10 times larger in size. By compiling published
properties of clump populations at similar spatial resolution between redshift
0 and 5, we find a tentative evolution of $\rm \Sigma_{SFR}$ and $\rm
\Sigma_{M_\star}$ with redshift, especially when very compact clumps ($\rm
R_{eff}\leqslant20$ pc) are considered. We suggest that these trends with
redshift reflect the changes in the host galaxy environments where clumps form.
Comparisons with the local universe clumps/star clusters shows that, although
rare, conditions for elevated clump $\rm \Sigma_{SFR}$ and $\rm
\Sigma_{M_\star}$ can be found.
