UE4 already uses HDR by default, and clamps it down to LDR using (mostly) the post processing settings. You can view the histogram by using VisualizeHDR. Also 4.15 added a filmic tonemapper that mimics the desaturation in film talked about at 22:10.
To add something to what Zac said, many tonemapper algohrythms are available on the internet and you can implement them in the material editor as a post process, if you are not satisfied with the built in one for some reason.
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