No, you cannot open a 1.5.x file in 1.4.x.
Nor exchange data in that direction.
Scribus -- like many other programs -- does not provide "forward" compatibility.
Theoretically, you can
- open the 1.5.x .sla file with a text editor,
- browse the xml soup to find out where the styles are defined,
- make a copy of your current 1.4.x file,
- open the copy with a text editor,
- browse the xml soup to find out where the styles are defined,
- paste the 1.5.x styles at the right place,
- tweak the definition of the new styles so that they look like the other ones in the 1.4.x document,
- open the 1.4.x file copy with scribus 1.4.x and hope for the best...
But you will probably be faster by recreating all the styles.
Of course, the other way round -- from 1.4.x to 1.5.x -- you should be able to import the styles.