We present a digital cell library optimized for 4.2 K to create controllers that keep quantum processors coherent and entangled. The library, implemented on a standard 40-nm CMOS technology, was employed in the creation of the first 4.2 K RISC-V processor. It has achieved a minimum supply voltage of 590 mV, energy-delay product of 37 fJ/MHz, and maximum operating frequency of 740 MHz, all at 4.2 K in continuous operation. These results have been obtained from stand-alone characterization, successfully executing small C programs/benchmarks at 4.2 K. The overall performance of the library compares well against the state-of-the-art libraries designed for room temperature. In particular, we compared the performance of the proposed library against a foundry supplied library for the same process in several combinational benchmark circuits, showing significant improvements in power dissipation and frequency of operation.