cos such a switch (from t4 to t2 dependent) requires an entry clearance check which has to be done in the applicant’s home country as per the pbs system under immigration rules.
http://www.ukba.homeoffice.gov.uk/sitecontent/applicationforms/pbs/dependants
Switching’ or moving into the route for Family Members Tier 1, 2 and 5
migrants
29. For family members of Tier 1, 2, and 5 applicants, switching is permitted when applying as:
• a family member who has or last had entry clearance or leave to remain as the partner or child
of a person who had leave under another category and who has since been granted, or is, at the
same time, being granted leave to remain as a relevant Points Based System migrant; or
• a child born in the UK to a PBS migrant and their dependant partner