Four card layout using flexbox

Hey, it looks nice! Some tips i can help you with:

HTML

• span heading: two heading tags would be better here, they give more meaning and accessibility :

<span class="head-1"> Reliable,efficient delivery </span>
 <span class="head-2">Powered by technology  </span>

CSS

• header: instead of using flex to center text you could use text-align on the header tag.
• <img> alt tags: as they're decorative you can leave the alt="" blank.

Other than that it's a really great job! I liked your decision to go with flex. Maybe try with Grid some other time and compare to see what you feel more comfortable with.