Mr McGuigan Sr, 53, was murdered at his home at Comber Court in the Short Strand area of east Belfast last Wednesday.
The father of nine's funeral has taken place. A 39-year-old man was arrested on Tuesday and two guns were found during searches in greater Belfast.
Three other men, aged 53, 44 and 41, arrested earlier remain in custody.
A fourth man has been arrested over the murder of former IRA member Kevin McGuigan.