Why is my samsung j4 plus is on bootloop?

It most likely seems to be the concurrence of several unrelated causes in combination with bad luck. I saw that it´s more common on model Samsung Galaxy J4+ (J415) than others and maybe it´s partially software bug also (in HOME CSC). Did you try to flash Combination ROM before? What was your battery level before the issue started?

In case you did flash the phone with Odin with the failed outcome and your battery level was desperately low, due to IMHO bad SW/HW design, you won't be able to charge the battery to run Download mode and flash the Stock ROM back.

I'm not sure if it's some kind of protection but especially if you try to flash Combination ROM, it probably somehow corrupts phone's firmware (I bet that it corrupt HOME CSC) which is indicated by the phone cannot boot and try fallbacks to the Download mode. Because of the low battery level which is not enough to start Download mode and phone power off. The main issue here is that due to corruption phone cannot charge the battery no matter if the USB cable is plugged in and to the charger. To fix this you must re-flash ROM and this is not possible!

The fastest way to fix this issue is to remove the battery and charge it externally to level enough to run Download mode and flash the Stock ROM.

Plug the USB cable to the PC and the phone and push and hold :

Vol UP + Vol DOWN + Power

The phone will start and no matter to restarts (maybe 15-20 restarts, approx. 15-20mins.) still hold all buttons until Download mode will appear on the screen. But this method didn't work for me.

EDIT 2020/05/17:

After further investigation and research I found solution which may work. In case your battery is charged and you still facing boot loop issue, you can try this:

At first you need to power off your phone (the state after failed boot ending with black screen is not powered off - in this case the fastest way how to power off the phone is to bypass the battery with guitar pick or another thin non-conductor - poker card, etc. otherwise you won't be able to proceed - DONT DISCHARGE YOUR BATTERY TO ZERO!) then connect first USB cable to the PC. Push and hold Vol UP + Volume DOWN (don't touch the Power button!) and connect USB cable to the phone. After a while you should see Download mode on the screen so you can release the buttons and continue to flash stock ROM.