  Telescope Array (TA) is the largest ultrahigh energy cosmic-ray (UHECR)
observatory in the Northern Hemisphere. It explores the origin of UHECRs by
measuring their energy spectrum, arrival-direction distribution, and mass
composition using a surface detector (SD) array covering approximately 700
km$^2$ and fluorescence detector (FD) stations. TA has found evidence for a
cluster of cosmic rays with energies greater than 57 EeV. In order to confirm
this evidence with more data, it is necessary to increase the data collection
rate.We have begun building an expansion of TA that we call TAx4. In this
paper, we explain the motivation, design, technical features, and expected
performance of the TAx4 SD. We also present TAx4's current status and examples
of the data that have already been collected.
