On August 24th, 2014, Hatsune Miku was confirmed to have another English database in production. Whether or not this was an update of Hatsune Miku V3 English or an English Append was unknown at the time.
On June 26th, 2016, more details regarding Miku V4 English have been announced via Wat's Twitter. He said that various aspects of Miku's V4 English voicebank were undergoing adjustments and being checked over. The tone of her V4 English voicebank would be aimed to sound similar to ORIGINAL ~ VIVID. The voicebank's bias of pronunciation depends, although it would have a bright/clear feeling.
In total the package offers the equivalent of 2 additional voicebanks achievable via XSY. The total tone variation offered by the package comes to a theoretical 4 voicebanks in total.
Phonetic notes as noted:
Most of her mispronounced phonetics were fixed and improved. However, the voicebank still has pronounciation issues in certain consonants, specially the [r], [l] and [l0] phonetics and the overall vocal performance mantains a notorious japanese accent.
According to some users, her phonetics sound clear only when working inside the optimum vocal range; in higher notes, they crisp and shrill, while lower notes sound mumbled. Extra edition has to be done to avoid this.
Software issues as noted:
According to producer Cillia, using the voicebank was a "challenging experience". She also mentioned that the voicebank quality and clarity is heavily affected when any kind of effects or filters are applied, making it not suitable for particular ways of mixing.
Although this vocal was created with a sound closer to the original japanese voicebank, there's still a timbre contrast between both libraries and switching languages during a song will be notorious.
Along with her English V3, she maintains one the shortest tempo ranges of any previous english vocaloids by Crypton Future Media (and english vocaloids in general).