Can I join two 4x4 fence posts?

I would avoid doing it. Get 8' posts and keep the 6' ones for other projects.

If you have to do it, I would do it at or above ground level. Presumably you're talking PT posts, which are treated on the surface but not inside. So the inside of any "join" will be less durable and you don't want it exposed to permanent moisture.

I've done it at my off-grid remote property, where you can't go to a lumber yard and get the right stuff. I McGyvered it 2 ways, intending it to be temporary, but 2 years later both ways still work.

1. Butt end to butt end with no cutting, but with a metal tie like this, since I had an extra one. https://www.homedepot.com/p/Simpson-Strong-Tie-BC-ZMAX-Galvanized-Post-Cap-for-4x-Nominal-Lumber-BC4Z-R/206059722 Note if you have to buy these, I'd just get the right-sized posts instead. Note you can get variety of steel connectors, say 2 flat long ones per post

2. With a half-lap joint with carriage bolts (don't use deck board screws, not strong enough).

Looking for a pic of the half-lap joints, I've come across https://backyardscape.com/how-to-extend-wooden-fence-post-height-step-by-step/ which describes both methods.

The good thing is that a 4' fence (unless e.g. has snow pushed against it) is actually fairly undemanding structurally, especially if it's attached to something solid at the ends. The bad thing is that it will still be a hassle to get it right, it will be weaker, and it will be less durable esp. in wet conditions, so don't do it unless you really have to!

You could do it if you have to doing a joint like the one shown below. Ideally, new posts would be desirable. You would want to keep this joint above ground and use bolts,washers and nuts that are stainless steel.

Is it possible, yes. Is it practical (.vs. just buying the right size posts), not really. Should it be below ground - no.

To really do it right you want a router bit or shaper cutter that will put tapered "fingers" on the end of the joint which interlock for maximum glued surface - which, unsurprisingly, is exactly what you'll see in the end joints of engineered lumber - but if making a 4x4 from engineered lumber, there would be 2 offset 2x4's that were end jointed, so that the end joints were not all at the same point on the resulting timber.

You can, of course, do it to a lower standard. It may work well enough, or it may fail.

Another option would be to:

Set steel posts and bolt the wood to them. Either cut them off or just let the posts stick 2 feet above the main fence (bolt them to the steel posts above ground level - don't half-bury them.) Depending on the purpose of the fence those extra 2 feet can be useful, i.e to string trellis wire for plants or monfilimaent to annoy deer above your solid fence.