I used to mess around a bit with radio controlled models ( including a hand made 1/12 scale r/c model of my 26 foot yacht) and I was wondering about using a LiPo instead of Li-Ion. We pill a lot of Amps out of LiPo, and a Universal Battery Eliminator Circuit (UBEC) can drop, say a 7.4V LiPo battery down to 5V and probably allow the 3A easily. I'll have to dig one out of the parts bin and check the specs. I'm pretty sure if the battery puts out more than 3 Amps it won't matter, The Pi should only draw what it wants up to its rated maximum.
Or maybe someone on the forum who is into robotics has already tried it?
Another alternative I could try is two Li-Ion power banks putting out 5V 2A each in parallel. I have half a sozen spare 10,000mAh and 20,000mAh power banks lying around and I have Y-cables with 2 x male, 1 x female USB. The worst I might do is start a small fire or explosion if the Y-cables short something in the power banks...
Of course, since I already have a Pi 3B+ and a 15" LED 12V TV as a monitor, I probably don;t need to bother. But that's the fun of having a collection of Pis - always something new to try
** EDIT **
The idea of using 2 x 2.1A outputs from power banks in parallel through a Y-cable to power the Pi 4 got me thinking. I noticed i had the LASER brand USB power adapter that I had used for the Pi 3B+ lying on the desk. A quick look on the label confirmed it is supposed to be 2 x 2.4A USB sockets with a total output of 5V at 4,8A.
Plugged the male ends of the Y-cable into the LASER adapter, Plugged a USB-C charge cable into the female socket of the Y-cable and the other end into the Pi and switched the power board ON.
All is running nicely. I suspect if I plug a couple of bigger USB 3 external drives into the Pi 4 USB 3 ports, I might fry the Pi 4 power circuit. But I suppose someone has to find out..
If it all works ok, that means I have a portable power source with enough Amperage to run the Pi 4 on the back of the portable monitor..
* I'm NOT suggesting people use a non-genuine power supply for the Pi 4. When I bought mine a few weeks ago, the 'official' Pi dealer in Australia didn't not have the 3A 'Raspberry' power supply listed, so I bought one as part of a kit that included a fan, on Amazon. There are thinks I don't like about the Vilross power supply, including that if I bump it, it cuts off power for a moment.