Full Idea
Given the Humean ontology, there is grave difficulty in making any sense at all of the concept of change with continuity as distinct from successive replacements.
Gist of Idea
Humeans can only explain change with continuity as successive replacement
Source
Harré,R./Madden,E.H. (Causal Powers [1975], 6. Intro)
Book Reference
Harré,R/Madden,E.H.: 'Causal Powers: A Theory of Natural Necessity' [Blackwell 1975], p.101
A Reaction
Hence the four-dimensionalist approach is basically Hume updated. The weird nature of time lurks behind this difficulty. If you can separate the moments of time, you can separate the bits of a continuous thing, and then ask how they relate.