  Hybrid superconductor-semiconductor Josephson field-effect transistors
(JoFETs) function as Josephson junctions with a gate-tunable critical current.
Additionally, they can feature a non-sinusoidal current-phase relation (CPR)
containing multiple harmonics of the superconducting phase difference, a so-far
underutilized property. In this work, we exploit this multi-harmonicity to
create a Josephson circuit element with an almost perfectly $\pi$-periodic CPR,
indicative of a largely dominant charge-4e supercurrent transport. Such a
Josephson element was recently proposed as the basic building block of a
protected superconducting qubit. Here, it is realized using a superconducting
quantum interference device (SQUID) with low-inductance aluminum arms and two
nominally identical JoFETs. The latter are fabricated from a SiGe/Ge/SiGe
quantum-well heterostructure embedding a high-mobility two-dimensional hole
gas. By carefully adjusting the JoFET gate voltages and finely tuning the
magnetic flux through the SQUID close to half a flux quantum, we achieve a
regime where the $\sin(2\varphi)$ component accounts for more than
\SI{95}{\percent} of the total supercurrent. This result demonstrates a new
promising route for the realization of superconducting qubits with enhanced
coherence properties.
