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For 1920x1080 pixels 4 ref frames are the maximum allowed at level 4.1. If your movie has black bars you can crop them though, to e.g. 1920x800 for a 2.40:1 movie. Then more ref frames are allowed while staying within the level 4.1 limits.
It is possible to force the encoder to write "level 4.1" into the bitstream, while in reality exceeding that limit, but it is HIGHLY discouraged. All you will get is a broken stream which will produce errors or not work at all in many players for what is probably a negligible increase in compression (and increasing ref frames is not the only way to increase compression anyways).
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