Hey, saw this earlier but just getting home. Weddings are high-stakes so i totally get the panic. Before you buy anything tho, are you shooting handheld or on a gimbal? The weight and size really matters for a long day and that a7iv can get heavy fast. I would suggest looking at these options for a $300 cap:

• Feelworld F6 Plus 5.5 Inch 4K HDMI Touchscreen is basically the go-to budget pick. Its very light which is nice for your setup, but be careful with the plastic mount because it can feel a bit flimsy if you are moving fast between the ceremony and reception.
• Portkeys PT6 5.2 Inch 4K HDMI 600nit is my top choice for color accuracy. The UI is way cleaner than Feelworld and it handles 3D LUTs better so you can actually see what your footage will look like. Make sure to check the ports tho... the a7iv has a full HDMI, but some monitors ship with micro cables that wont fit your camera.
• Desview R6 UHB 5.5 Inch 2800nits is what you want if any part of the wedding is outdoors. Its incredibly bright so you dont need a sunhood. Just a heads up, that extra brightness will drain your NP-F batteries much faster than the others. Make sure to pick up a solid cable like the Kondor Blue Braided Full HDMI to Full HDMI 12-24 Inch because cheap cables are usually why people see lag or signal drops. Let me know your rigging setup and i can help more!

literally me right now. i have an a7iv and trying to nail focus on that tiny screen during a live event is basically impossible... i am also hunting for something under 300 bucks because gear prices are getting insane lately. i feel your pain with the wedding deadline too, that pressure is the worst. agreeing so much with the point about weight. i borrowed a heavy setup once and my wrist was killing me after two hours of handheld work. when you are on a budget you usually end up with heavy plastic or bulky batteries so it is such a tough balance to find something light. hope you find something that works in time for the weekend because the panic is real lol.

^ This. Also, keeping the weight down is so important for those long wedding days! You're gonna do amazing and getting a monitor is seriously such a fantastic move for your workflow. Honestly tho, you should definitely just search for best budget a7iv monitors on YouTube real quick. There are some incredible videos where people actually test the outdoor brightness and lag side-by-side. I saw one a few days ago that was basically a lifesaver for choosing cables too... literally just look up a7iv wedding rig and it should be one of the top results. Also check the r/sonyalpha subreddit because they have a whole thread on which batteries actually last through a full reception without dying every twenty minutes! People there are super helpful with reliability tips. TL;DR: Go watch a couple YouTube comparison videos for daylight visibility tests and check Reddit for battery life complaints before you hit buy tonight!

Like someone mentioned, you really gotta watch the weight, but I worry more about the connection stability. If that HDMI cable wiggles even a tiny bit, your screen goes black right when the kiss happens. Be careful with those cheap thin cables that come in the box. I would suggest picking up an Atomos AtomFLEX Pro HDMI Coiled Cable because they actually stay seated in the port. For the monitor itself, the Portkeys PT5 II 5 Inch 4K HDMI Monitor is a pretty safe bet for reliability in that price range. It feels a bit more robust than some of the other plastic options. Also, make sure to bring way more power than you think you need. Those external screens eat through juice way faster than the camera does, and you dont want to be swapping batteries mid-vows. Stick to the Sony NPF-970 Rechargeable Battery Pack or a very high-quality clone because the cheap ones can die without warning.