Response to Hi, Thomas. This top manifold will not fit the C-4000 filter, you will need the Top Closure Plate (C3000/4000) - CX3000DA. The filter plug as you call it is 3".

An image is worth 1,000 words! Add an image or two to your comment to help ensure it is understood accurately.

Yes, please email me exclusive discounts, swimming pool tips and helpful how to guides. This comment may be posted on a public site. Do not include: HTML, references to other retailers, pricing, personal information, or any profane, inflammatory language.