STM32 UART boot-loader Go command is unclear AN3155

Hello,

I am implemented UART bootloader on my windows controlled test bench. All is great and working, but some uncertain exist with jump command.

Its says in app note:

jumps to the memory location programmed in the received ‘address + 4’ 
(corresponding to the address of the application reset handler).
For example if the received address is 0x0800 0000, the bootloader jumps to the 
memory location programmed at address 0x0800 0004.
In general, the host must send the base address where the application to jump to is 
programmed.

My understanding is that my program starts at 0x0800 0000. So I should jump to 0x0800000-4 ?

If I do so, I get NACK, and nothing happens. But if I jump to 0x0800000, all it's good. My program seems to be running, but based on application note, it means I am jumping to 0x08000004 address

So what is in that first 4 bytes of data at the start of the flash It says reset handler, so I suppose to jump over this point ? So this is why I am jumping just a bit further into flash, since bootloader already does this for me ?

In app note:
-> initializes the main stack pointer of the user application

Any insight will be welcome !