OK. weird stuff can happen with TIFF files that include adjustment layers.
It might load fast but PL4 really doesn’t like that image. Here’s a screenshot of the tone curve palette
As you can see the upper limits are no longer 255 but 6502!!! What’s more, what you can do with the curve is very limited and you can’t reset it.
I tried loading a TIFF without any additional layers and it works fine.
@StevenL I think I have just discovered a bijou buguette 
That is, unless PL was never meant to handle such files?