Hampshire will host Glamorgan, while Somerset travel to Nottinghamshire in the semi-finals of the YB40.
Glamorgan pipped Northants after a win against Yorkshire saw them qualify from Group C as the best second-placed side.
Champions Hampshire beat Derbyshire to top Group B, while Group A winners Notts also get home advantage after victory over Kent.
Somerset make up the quartet after finishing top of Group C with a win over local rivals Gloucestershire.
Hampshire and Glamorgan will meet at the Ageas Bowl on Saturday, 7 September (12:45 BST), while Notts will face Somerset on Monday, 9 September (16:10 BST).
The final will be played at Lord's on Saturday, 21 September.