The "grabbers" you refer too aren't something you need to worry about. The printer (the person, not the machine) will print your artwork on sufficiantly oversized stock. All you need to worry about is supplying your artwork with enough bleed so that your print can be trimmed to the correct size after printing. The amount of bleed you need is something you should discuss with your printer (again, the person), different printers require different amounts.
If you don't know before hand then a good starting amount is 3mm or 1/8 inch (possibly more, but certainly no less than that—too much bleed is no problem, not enough will be).
You can find a much more in-depth explination of bleed in this previous Q&A:
How can I determine how much bleed to use?
Not entirely relevant, but if you're using an A4 preset in Photoshop you can set up some guides for your bleed pretty easily:
Pull some guides to each edge of your page (they should snap in to place)
Increase your canvas by the required amount. Setting the numbers to relative makes this much easier. For a 3mm bleed add 6mm to each axis.
- You now have guides set for your work area. Your artwork should extend all the way to the edge of the canvas, but anything outside of the guides will be trimmed.