Cheapest way to dissipate energy (discharge battery)
It seems you want a dummy load that can dissipate 800 W.  Normally I use incandescent light bulbs for things like this.  They have some nice properties:

Are designed to handle the heat.

Are cheap and plentiful, so a combination at the desired power can usually be found.

Are self-indicating.  You know when they are on or off just by looking.



However, there are two problems with this.  Incandescent light bulbs are getting less and less available nowadays, and are even banned in some cases.  4 V is also a difficult voltage for dissipating significant power.  Even 12 V automotive bulbs aren't going to dissipate significant power at 4 V.

So in the end you probably end up with a bunch of power resistors in parallel.  The big fat ceramic wire wound types can do this.

There are also things called electronics loads.  Those are nice, often dump the power back onto the power line, but are expensive.