Any MTB trigger shifters that work with Shimano RD-4600? Are there any MTB style flatbar trigger shifters that are compatible with Shimano 4600 road components?
The Shimano SL-4600 are flat bar shifters for the 4600 road components.

SL-R780 and SL-R460 shifters are also listed by Shimano as compatible, so you have a couple more options than I first thought. 

IIRC mountain bike 10 speed shifters use a different pull ratio so all are off the table, which you probably know hence the question.

Even the new 4700 shifters use a different pull ratio to 4600 so don't be tempted to use the SL-4700 version.