Meanwhile i got some hints after few hours of reading comments and arguments on this forum.
The GPIO pins include 5v Input, so its likely to “work” with the pijuice HAT - Though its unclear the max amps input on the GPIO pin of RockPI 4, Also pijuice would drive max 5v 2.5A while RockPI would need at least 5v 3A
As it competitor RK3399 based SBC with the numbering 4, the board consumes around 15-18W
Though the powering of this board is a bit complicated at 1st, it can be simple with the Quick Charge 3.0 (Now i understand that QC would self regulate the voltage)
So looking at this principle, i found a charging module (IP5328) with QC3 enabled, the spec seems to fit:
Input voltage 5-12V (fast charge protocol automatically adjusts the voltage) Type-C input
charging current; 3100mah (fast charge protocol automatically adjusts the current)
output voltage is; 5V-12V ± 0.3V (fast charge protocol automatically adjusts the voltage)
the output USB rated current is; 3.1A (fast charge protocol automatically adjusts the current)
I also found 28W 5v Solar panel with QC 3.0 enabled (using Type-C Output) with max output of 5v 4.8A.
My next search will be for a suitable battery, taking in consideration physical measures (that would fit in my setup under the 7" screen) and battery capacity as well as charge/discharge capacities.