Can a standard A4 sanding page be cut into 4 pads that will fit the Festool 575382 RTSC 400 sandaer

This sander's pad is 80mm x 133mm, or roughly 3-5/32" x 5-1/4". You can certainly use any type of hook and loop sandpaper with this sander, however you would lose one of its best features which is the great dust extraction. I would suggest you look at Festool abrasives' pricing which is competitive and tend to last longer than nearly every other brand. So, while the cost per sheet may be a little more in some instances, you will use more sheets of another brand to do the same amount of sanding, thus costing more in the long run.