Four tonnes! For such a quantity, you should buy at a gravel plant - if you can live with a limited sortiment of colors. There's a mine about 25 km from you in Weiach, they sell gravel for 40-50 Fr./tonne. I don't know if they'd transport it for you and how much that costs. Look at page 14 of their price list PDF for detailed information about their gravel offers. Gravel from Germany probably is even cheaper.

Maybe you can find better prices for plants at the Migros. At the popdown menu named "Folgende Filialen, Unternehmen anzeigen", you can search for outlets with "Do It + Garden" section.

The Ikea in Dietlikon sells cheap plants, but last time I was there, they mostly offered succulents and potted plants of all sizes, like this one. They also sell decorative colored gravel, but only in sub-kilogramme bags.