Why ProRes 4444?
ProRes 4444 is designed for post-production workflows that need high color fidelity and an alpha channel. It preserves soft hair, motion blur and partial opacity more appropriately than an ordinary H.264 MP4.
Why are the files large?
It is an editing codec, not a compact web-delivery format. The larger bitrate makes playback and recompression inside professional tools more predictable, but it also means storage and transfer costs are higher.
Using it in an editor
Premiere Pro and After Effects
Import the MOV, place it above the background layer and verify that the clip is interpreted with a straight or premultiplied alpha setting appropriate to the export.
DaVinci Resolve and Final Cut
Place the cutout above the destination footage. If dark or bright fringes appear, inspect alpha interpretation before assuming the removal mask is wrong.
Do not serve it as a website asset
ProRes 4444 is unnecessarily heavy for normal web delivery and is not the compatibility answer for browsers. Use transparent WebM or an Apple-oriented HEVC-alpha alternative for playback.
Availability here
Every transparent video result here can also be exported as ProRes 4444 for free (up to 15 seconds), and the WebM to ProRes converter does the same for a WebM you already have. Do not rename a WebM file to MOV and expect an editing master.