^ This. Also, unfortunately, that store advice is really disappointing to hear. I had issues with the Sony Alpha 7 IV for birding because the AF tracking isnt as good as expected compared to the newer AI-driven systems. The Sony Alpha 6700 actually has a dedicated AI processing chip that helps lock onto eyes much faster, which is huge for macro work where depth of field is razor thin. If you go full frame, you're losing that 1.5x crop reach on your Sony FE 200-600mm f/5.6-6.3 G OSS lens, so youd have to crop in anyway and lose megapixels. The a6700 is basically a baby a7Rv in terms of brains. Since it finally uses the Sony NP-FZ100 battery, your power issues are solved too. Grab a fast Sony SF-G64T Tough SDXC card and you are set for way less money than the full frame setup. TL;DR: Get the a6700. It has the newer AI autofocus chip and gives you more reach for birds and bugs than the a7 IV at a much lower cost.

Honestly, that camera store guy is kinda steering you wrong. Saying the a6700 is just for video is a massive oversimplification. Like someone mentioned, sensor size is a factor, but for birding and macro, the tech inside the Sony Alpha a6700 Mirrorless Camera actually gives it an edge. I switched from a full-frame setup to the a6700 for my wildlife trips specifically because of the dedicated AI autofocus chip. It tracks bird eyes way better than the Sony Alpha a7 IV Mirrorless Camera does. Plus, your Sony FE 200-600mm f/5.6-6.3 G OSS gets that 1.5x crop factor, basically giving you more reach without needing a teleconverter. For your macro work, the APS-C sensor actually helps get a bit more depth of field at similar magnifications, which is a godsend when you're shooting tiny stuff. Both cameras use the Sony NP-FZ100 Rechargeable Battery, so you're getting a huge upgrade in shots per charge regardless of which one you pick. TL;DR: The a6700 has the newer AI autofocus and better reach for birds, making it the smarter technical pick for your specific lenses.

Based on your requirements for underwater macro, land macro, and bird photography, and the need for improved autofocus (AF) performance and battery life compared to your current Sony a6300, here are my thoughts on the Sony a6700 vs Sony a7 IV:

1. Sensor size: The Sony a7 IV features a full-frame sensor, while the a6700 has an APS-C sensor. The larger sensor of the a7 IV can provide better low-light performance, wider dynamic range, and shallower depth of field, which can be advantageous for macro photography.

2. Autofocus performance: Both cameras are likely to have improved AF compared to the a6300. However, the a7 IV, being a higher-end model, may have a more advanced AF system, which could be beneficial for tracking birds in flight.

3. Battery life: The a7 IV is likely to have better battery life compared to the a6700, as full-frame cameras generally have larger batteries. This could be particularly useful for longer shooting sessions.

4. Lens compatibility: The a7 IV, being a full-frame camera, will work best with full-frame lenses. While your current lenses (90mm macro and 200-600mm) will work on the a7 IV, they may have some vignetting or reduced resolution due to the crop factor. The a6700, being an APS-C camera, will be fully compatible with your existing lenses.

5. Price: The a7 IV, as a full-frame camera, is likely to be more expensive than the a6700. Consider your budget and whether the additional features of the a7 IV justify the higher cost.

Considering the camera store's advice and the points mentioned above, the Sony a7 IV seems to be a better fit for your needs, particularly if improved AF performance and battery life are your primary concerns. However, if budget is a constraint and you prefer to use your existing lenses without any compromises, the a6700 could still be a good choice.

Ultimately, it's best to try out both cameras, if possible, to see which one feels more comfortable and suits your shooting style better.