Slowly Changing Dimensions Type 4 SCD type 4 keeps the foreign keys to the original dimension and the mini-dimension in the fact table.
Why is the foreign key to the mini-dim not stored in the original dim?

Link to an article explaining SCD types: http://www.kimballgroup.com/2013/02/design-tip-152-slowly-changing-dimension-types-0-4-5-6-7/
Usually, we use SCD-type-4 when a dimension(SCD Type 2) grows rapidly due to the frequently changing of its attributes. These frequently changing attributes will be removed from the main dimension and added in to a new one known as Mini-Dimension.

==> Mini Dimension do not store the historical attributes, but the fact table preserved the history of dimension attribute assignment

A good example is illustrated here : link-SCD4