Does Deckorators either have a post cap that will fit the 4x4 vinyl sleeve or an adapter.

Hello Commander, Yes, but let's start with the post sleeves from Deckorators, available in white or black and two sizes: Deckorators 4-in x 4-in x 39-in Black Aluminum Deck Post Sleeve, Model #117663, and the Deckorators 4-in x 4-in x 52-in Black Aluminum Deck Post Sleeve, Model #118444.For a black post cap, you might consider the Deckorators 4-in x 4-in Black Aluminum Deck Post Cap, Item #587113, Model #117677. for a white or black post cap with solar light, please consider the Deckorators 4-in x 4-in Solar VersaCap White Solar LED Plastic Deck Post Cap, Item #254599, Model #82191. If you need further assistance, please do not hesitate to contact our knowledgeable Customer Service Team at 800.445.6937. Thank you. Your Deckorators Team