For 15.4 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:cartwm:branches:devel:libraries:c_c++/15.4/home:cartwm:branches:devel:libraries:c_c++.repo
zypper refresh
zypper install boost-gnu-openmpi4-hpc-python3
For 15.3 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:cartwm:branches:devel:libraries:c_c++/15.3/home:cartwm:branches:devel:libraries:c_c++.repo
zypper refresh
zypper install boost-gnu-openmpi4-hpc-python3