Can I quick detonate C4 while another weapon is equipped?
Solution 1:
No, C4 can only be detonated by the thrower using the fire button or by yours or enemy bullets. C4 could be detonated on MW2 by double-tapping reload, but no such shortcut exists in Battlefield, likely because of their desire to make the game 'realistic'.
Solution 2:
Also keep in mind that explosive weapons detonate C4. It is possible (and very handy) to use a shotgun with explosive rounds when you're C4ing things, as this allows you to fire at your C4 to detonate it.