Ultrafast structural dynamics in the condensed phase represents a key topic of current physics, chemistry and materials science. Femtosecond hard X-ray pulses are important structure probes that have been applied in time-resolved X-ray absorption and diffraction(1-4). Optical pump/X-ray probe schemes with compact laser-driven table-top sources(5-11) have allowed for tiny changes of diffracted intensity to be measured with X-ray photon statistics, which has set the ultimate sensitivity limit(12-14). To address the strong quest for a higher X-ray flux, here we present the first hard X-ray plasma source driven by intense mid-infrared sub-100-fs pulses at 3.9 mu m. The comparably long optical period allows for accelerating electrons from the Cu target to very high kinetic energies and for generating a characteristic K alpha flux of 10(9) photons per pulse, 25 times more than with our 800 nm driver(9,10). Theoretical simulations account for the experimental results in a wide range of driving fields and predict a further enhancement of X-ray flux.