You could do that, and it would technically work, but that is a big blowup for the 4:3 material and it would probably lose a lot of resolution in the process.
There's a tutorial
here on how to mix 16:9 (1024 x 576) and 4:3 (720 x 576) footage in projects. You are pushing it even further by going to 1440 x1080, but it might be helpful.
Also, you might want to look at Shane's
method for upressing via Compressor. Some mathematical adjustments will apply for 1080, and it takes a LONG time, so best used for a couple of shots only, but might be handy.