Finally, Raspberry Pi 4 is released. It got 3x higher performance 1.5GHz quad-core 64-bit ARM Cortex-A72 CPU, and you can choose 1Gb, 2GB and 4GB. That is very useful if you are doing machine learning. Moreover, it supports:
- Full-throughput Gigabit Ethernet
-Dual-band 802.11ac wireless networking and Bluetooth 5.0
-Two USB 3.0 and two USB 2.0 ports
- Dual monitor support, at resolutions up to 4K
- VideoCore VI graphics
- 4Kp60 hardware decode of HEVC video
That is more powerful computer. I think even you can use it to replace your home desktop. For basic model (1gb RAM) $59.95 and the top model (4GB) $94.95.

But the down side, it sounds more power is required. The input power is 5V 3A USB-C. Moreover, that is much more expensive.