Those are "compressed" files -- they need to be uncompressed into "video space" and thus your going over the total amount of memory -- Note that it is 4GB for Media Store 1 and 3, and then it is another 4GB for Media Store 2+4. Tip for you would be to render the file to the size you need -- i.e. since you say they are lower thirds don't render as Full Raster but only the size that they need to be. Then when you load them you will be able to use the Media Properties to position them.
Usually what I do to ensure position is correct is that I load a single Full Frame of the Lower third as a sample. Then I load the vertically reduced images. You put the on frame into say Media 2/4 so that you can put that on the background -- load in the animation one and then key it over top. Use the parameters to move it to the correct location. Save this and now any time is loads it will use these new parameters.
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