A common subversion to Numbered Sequels. As the number of installments of a series goes up, the less likely it has a number behind.
Usually the series and sequel titling go in the following order (although only some series go through all five steps):
First Installment
Numbered Sequel
Numbered sequel + subtitle (or 3D).
Subtitle
Recycled Title
At this point, anything can happen, including [...] [tvtropes.org]