one does not simply miku v4 english
Anyways, all of that aside. Having worked with Miku v4 for (quite) a while now, some of the more universal things I've done that specifically pertains to Miku for a song like you have stated were:
- make sure that you manually edit the phonemes; Miku struggles with that sound that's somewhere between [@], [V] and [Q] and (as stated before) and the [e]'s. (you were already doing that... right?)
- *slightly* increase the gender parameter (on average I increased the value by around 10, occasionally a little less)
- start with a slightly higher dynamic (~+5 to 15), and adjust from there dependent on the part of the song you're on
- XSY with Miku v3 if you have that
Hope this helped!