  Superconducting qubits with intrinsic noise protection offer a promising
approach to improve the coherence of quantum information. Crucial to such
protected qubits is the encoding of the logical quantum states into
wavefunctions with disjoint support. Such encoding can be achieved by a
Josephson element with an unusual charge-4e supercurrent emerging from the
coherent transfer of pairs of Cooper-pairs. In this work, we demonstrate the
controlled conversion of a conventional charge-2e dominated to a charge-4e
dominated supercurrent in a superconducting quantum interference device (SQUID)
consisting of gate-tunable planar Josephson junctions (JJs). We investigate the
ac Josephson effect of the SQUID and measure a dominant photon emission at
twice the fundamental Josephson frequency together with a doubling of the
number of Shapiro steps, both consistent with the appearance of charge-4e
supercurrent. Our results present a step towards novel protected
superconducting qubits based on superconductor-semiconductor hybrid materials.
