Hi Phil,
You are slower than what the guidelines suggest is desirable, but as long as your trend is down, you should be fine. There are many patients who respond slower than you do and ultimately achieve MMR. We call them turtles and they do very well indeed. I was a turtle - until I switched drugs!
What is your FISH level? This is an actual measurement of bcr-abl cells under the microscope and is much more a prognostic indicator. You want to be zero FISH at 12 months which is complete cytogenetic remission. At 12.5% PCR at 4 months, you are doing o.k. - and will likely meet this goal. Taking more imatinib (i.e. 600 mg daily) could very well accelerate your downward trend. Imatinib is a dose dependent drug upwards to 800 mg. It does increase side effect potential at higher dose.
In your case, I would switch to dasatinib. Dasatinib is a proven first line treatment alternative with a deeper response profile than imatinib. Also - having had imatinib first, data is suggesting that switching drugs can give a 'one - two' punch to CML by targeting other higher order cells in the CML hierarchy and may gvie you an even better response than either drug alone.
I would not start dasatinib at the current full dose of 100 mg. In your case, 50 or 70 mg should be started and your trend monitored monthly. You could very well respond dramatically downward on a lower starting dose. It depends on whether your doctor is aware of dosing strategy when it comes to CML drugs and dasatinib in particular.
You could always switch back to imatinib or nilotinib if dasatinib does not work out.