Are power banks compatible with 4G hotspot devices?

Yes. But it depends on the type of 4G hotspot device.

Power banks are typically designed to charge 5 volts mobile gadgets with USB charging ports. So, before you buy, it is important you check the product specification of the hotspot gadget you wish to buy. Few key things to look out for are:

1. Battery mAh rating: This will help you decide on the type/capacity of power bank you should buy.
2. Type of power bank: I would suggest you go green and buy a solar power bank charger that will recharge your hotspot device at least twice. To determine this just divide the mAh rating of the power bank by that of the hotspot gadget. This will indicate how many times the power bank will recharge your mobile gadget from zero to 100% before it runs out.
3. There are too many fake products in the market therefore you need to also look out for a good brand name like HetoGrow, supported by meaningful user guidance notes, warranty, and product quality spec. It is important you read through these to understand what you are spending your hard earned money on.
4. There are other factors to consider. I encourage you to invest a few minutes to read the HetoGrow Quality Assurance page which is regularly updated with new tips to look out for including explanation and implication of volts and mAh, etc.