ngl, for a high-stakes grad ceremony, you need gear that wont fail you! I love these two for pure reliability:

• Fujifilm Fujinon XF 50mm f2 R WR: This thing is amazing because the autofocus is basically instant and its fully weather-sealed. Total peace of mind if the weather turns bad!
• Fujifilm Fujinon XF 90mm f2 R LM WR: The background separation is fantastic and the quad linear motors mean it never misses focus. Its incredibly sharp and rock solid!

Like someone mentioned, blur is key, but are you shooting indoors or outdoors? I'm satisfied with Viltrox 75mm f1.2 AF Fuji X for reach and Sigma 56mm f1.4 DC DN Contemporary Fuji X for autofocus.

Gonna try this over the weekend. Will report back if it works!

Listen, if you want that creamy background blur aka bokeh that makes people go wow, the 56mm is absolutely the king of the X-series! I have spent so many hours with that lens and the way it renders skin tones is just fantastic. But, if focus speed is driving you crazy like you said, you gotta be careful with the older version of that lens because it can hunt a bit. Since you have a deadline and a 700 dollar budget, here are my top picks for your XT4 right now:

• Fujifilm Fujinon XF 56mm f/1.2 R
• This is the bokeh monster you want. You can definitely find one used in great shape under your budget. It is an 85mm equivalent which is basically the gold standard for portraits!
• Sigma 56mm f/1.4 DC DN Contemporary for Fujifilm X
• Honestly, this might be your best bet. It is way faster at focusing than the original Fuji 1.2 and it is incredibly sharp even wide open. Plus it is way cheaper than the Fuji version.
• Fujifilm Fujinon XF 50mm f/2 R WR
• Super snappy focus and very sharp. It is cheaper and smaller, tho it wont give you that crazy thin depth of field like the 1.2 does. Ngl if your sister moves around a lot, that Sigma is a total lifesaver. The 18-55 is okay for general stuff but it just doesnt have the soul of a fast prime lens for grads. Youre gonna love the difference immediately! Just make sure your eye AF is turned on in the settings... it makes a world of difference when she is moving her head.

Popping back in because i have been through that exact same nightmare with the X-T4 focus hunting at the worst times. Honestly, it is so demoralizing when you are trying to capture a memory and the camera just keeps pulsing back and forth... it really ruins the mood of the whole shoot. Sessions where i felt like a total amateur because the gear just wouldnt cooperate were the absolute worst. If you want to avoid that headache, you gotta look at the linear motor lenses. The glass is heavier usually, but it actually works for people who move around:

• Fujifilm XF 90mm f/2 R LM WR: This is the real deal. The background just melts away because it is a longer focal length, so you get that creamy look without the f1.2 price tag.
• Fujifilm XF 50mm f/2 R WR: Way more affordable and the autofocus is basically instant compared to the kit lens. Focusing on just the aperture is a bit of a trap. A lens with a slower f2 aperture but a longer focal length often gives you better blur and way more reliability than a fast lens that cant find the eye to save its life. Keep some of that 700 in your pocket and go for something with better motors.

• Honestly been thinking about this for a few hours because I am in the exact same boat with my X-T4... it is basically the most frustrating thing I have dealt with in a while.
• I have been shooting for years and usually know my way around gear, but I still cant get that clean background separation with this setup, it just looks like a cluttered mess every time tho.
• I even tried some DIY diffusion hacks and light modifiers to see if I could mask the busy background myself but the focus still hunts constantly when my subject moves even an inch... been trying to figure this out for months and I still dont have any answers.