Why does my phone show a warning when I install V4?
Because V4 isn't on Google Play or the App Store. Phones warn about any app that comes from outside their own store, and that warning is your phone doing its job.
This site doesn't walk you through switching protections off. Read what the prompt says and decide for yourself. If you're not comfortable, don't install, or use a spare phone or tablet that doesn't hold your banking apps. The safety page covers the real risks.