You'll need a modifier in there for the first detonate, but you could try something like the following (might need tweaking, this is not something I've tested in game):
#showtooltip
/cast [mod:ctrl] Wild Mushroom: Detonate;
/castsequence reset=5 Wild Mushroom, Wild Mushroom, Wild Mushroom, Wild Mushroom: Detonate
If you want to pop them early you hold down your modifier, otherwise it will plant 3 mushrooms and then detonate on the last press. You might need to play around with the reset time, if you tend to plant mushrooms in advance you might want to set this a bit longer so that you don't have to use the modifier key to pop them.
I also like that this thread has been necro'd twice :)
Edited, Nov 4th 2011 5:59am by Tynuv