The 4X burners are real picky about maintaing the data rate. For instance, on an internal drive they often will not burn 4X unless DMA transfer is enabled.
I would start by diagnosing your connection issues. Maybe something is causing a slow down there, so your drive is not unlocking the 4X speed.
Scan your media with one of the tools listed above in the stickies. If your firmware updated properly, then your media should pass for 4X burning. If not, doublecheck your firmware update. There were some later revisions of the Pioneers that didn't want to work with some of the firmwares--you might need to investigate this further as well.
Keep us posted on what happens.
If it ain't broke don't try to fix it! But, hell if you can tweak that bastard just a little bit more....hehehe.