The other thing to consider (and budget for) with a 600/4 is support. It’s not a light lens. Personally, I can shoot handheld with mine, and the MkIII/RF version is a bit lighter. But by ‘shoot handheld’ I mean walking with it carried on a BlackRapid lefty strap and raised for an occasional shot, as I do when shooting passerines.
If I’m going to be standing in one place for any length of time, I use a monopod with a tilt head (I carry that on a belt when hiking). If I’m going to be standing in one place for a long period of time, such as when shooting raptors in winter, I use a tripod with a gimbal head (you don’t want to use a ballhead with a supertele lens).
You’re probably aware of this, but with tripods and related items, the 2/3 rule applies: light, stable, and expensive…you can only have up to two of those three. For a good quality tripod/gimbal setup, e.g., Gitzo legs and a Wimberly gimbal, you can expect to spend north of $1500. My Really Right Stuff setup (3-series legs, leveling base, side gimbal, monopod and tilt head) was around $3000, plus an extra $500 for a BH-55 ballhead I can easily swap for the gimbal for regular lenses.