Photoacoustic microscopy (PAM) offers label-free, optical absorption contrast. A high-speed, high-resolution PAM system in an inverted microscope configuration with a laser pulse repetition rate of $100,000\mathrm{Hz}$ and a stationary ultrasonic transducer was built. Four-dimensional in vivo imaging of microcirculation in mouse skin was achieved at 18 three-dimensional volumes per second with repeated two-dimensional (2D) raster scans of 100 by 50 points. The corresponding 2D B-scan (50 A-lines) frame rate was $1800\mathrm{Hz}$ , and the one-dimensional A-scan rate was $90,000\mathrm{Hz}$ . The lateral resolution is $0.23\pm 0.03\mathrm{\mu m}$ for Au nanowire imaging, which is 2.0 times below the diffraction limit.