I've spent a lot of time behind the 5D Mark IV and let me tell you, it's a workhorse, but it can be picky with some third-party flashes. I used to carry around a massive bag of Eneloop AA batteries for my older Speedlites. It was such a hassle. I remember one wedding where I was frantically swapping batteries during the toasts because my recycle time had crawled to five seconds. Not a fun experience when the couple is crying and you're staring at a red light waiting for a beep. Switching to units with lithium-ion packs changed the game for me. The Godox V1-C Flash for Canon is actually pretty solid on the 5D IV. The TTL is consistent enough for wedding work, though I find I have to dial in a bit of flash exposure compensation when I am bouncing off high or dark ceilings. The round head gives a nice falloff that looks a bit more natural than the harsh rectangular beams. If you are worried about the Godox being buggy, here are a few things to consider:

• Godox V860III-C Speedlight for Canon is a great alternative if you dont care about the round head. It uses the same battery and is very reliable.
• Canon Speedlite 430EX III-RT gives you that native peace of mind but youll miss the fast recycle times of the lithium batteries.
• Godox XPro-C TTL Wireless Flash Trigger is worth picking up if you ever want to move that flash off-camera for portraits. Honestly, the Godox system is just more flexible for the price. Just make sure you update the firmware right away and you should be fine for your Boston shoot.

Tbh weddings in Boston can be a nightmare with those dark reception halls, so you really need something that wont quit on you. I think I heard that some third-party units have weird communication lags with the 5D IV's E-TTL system, especially when the battery levels drop. Not 100% sure but someone told me the pins on some cheaper mounts can actually wiggle loose during a long day of shooting. You might want to be careful about the Li-ion vs AA debate too. IIRC, the Li-ion packs are way faster for recycling because of the higher voltage, but I've heard they can sometimes thermal shut down without warning. Make sure to check the duty cycle specs before you buy. I would suggest going for a used OEM unit if you can find one in your budget, as the weather sealing usually matches the 5D bodies better.

Seconded!