Hello
Please refer to the HP Post Error Message guide found at this link.
Scroll down to front panel codes.
Four LED blinks could mean power supply adapter has failed. It is also possible the surge from lightning has daamaged the power supply.
If you are good to work in case then order adapter and/or power supply replacement from HP. I really think power supply is fried.
Or contact HP for service if you are in warranty. There are three different model for this PC (Ultra Slim, SFF, and Convertible). Which one is yours? The HP support site for SFF can be seen here.
Grzy