Consider the Godox V1 for on-camera flash, but strongly recommend a trigger with an external flash like the Godox AD200 for better options and results. On-camera flash suits quick event photography, while the AD200 with a modifier excels in other scenarios, offering superior performance.

Jumping in here because while Godox is popular, you might want to consider the high-end stuff if you are doing professional events with an a7R IV. I have seen some weird connection issues with cheaper feet on that specific multi-interface shoe, so be careful there.

• Sony HVL-F60RM2 External Flash: This is the safest bet for reliable AF integration. Since it is native, it talks to the camera better than anything else. Fast recycling too, but I would suggest keeping an eye on the heat if you are firing rapid bursts all day. It can occasionally throttle to protect the electronics.
• Profoto A10 AirTTL Sony: If you want the absolute best consistency, this is it. The color temperature doesn't drift like the cheaper brands do. Build quality is basically like a tank, though it is obviously way more expensive.
• Westcott FJ80 II Speedlight: I have been using this one lately and the touchscreen UI is actually super helpful when you are in a rush. It is a bit more stable than the budget options but still saves you some cash compared to the Profoto. Make sure to check the pins on the shoe regularly tho, the Sony mounts can be a bit fragile if you bump them during a busy event.

Get Godox V1 PRO, it is more powerful than V1.

If you are looking for off camera flash, you can get the new released Godox AD200Pro II now.

Godox V1 is one of the best options for a7r IV.

Also recommended GODOX V1. https://www.bhphotovideo.com/c/search?Ntt=Godox+V1+S+Camera+Flash+Speedlite&BI=8941&KBID=10361&SID=12345&DFF=d50