Two Ways
****First 1****: Turn Your Device Off,Then Plug IN The Micro Usb cable to PC or laptop,Next Hold Down The Volume Down Key Then wait 5 secs Then Plug The usb Into The Phone While still Holding down The Volume down key You Should Realese As Soon As You See Fastboot iT is A Black Screen That should Say Fastboot For Are Device. ****Second 2****Launch Adb On Your Computer Then Type in "Adb Reboot Bootloader" With Out the Quotes! ...... I Am guessing your Trying to flash Recovery T.W.R.P If So Get The folder where Your Adb And Fastboot Is in Then Put The T.W.R.P Image In There. To Make It Eaiser Rename The Image Recovery,Then Boot Your Device Into Fastboot Then Type Fastboot Devices If Your Device shows Up Type "fastboot flash recovery recovery.img" With Out Quotes!! Then If Its Done The process Right You Should Have T.W.R.P. "###Do This At Your Own Risk Since you have a Zv4 I Have Only Tested This On Zv3 I Doubt It Would Do Any Harm But Just A Warning ...You Never no###"Remmber That It Has Some Bugs That I need TO Fix But I Am soft Brick Because of My Boot.img.. The Twrp Cant Detect Are Devices Partions...
..Which means You Can Not backup Applications ETC.