*DO NOT* pay $4,000 to get your book published. That is an ungodly sum, which you will never make back through self publishing.
If you really do believe in your book, keep trying with traditional publishing.
But, instead of submitting directly to publishers, query agents who represent the type of book you have written. An agent will have much more success selling your book to a publisher than you will. They have personal connections, know much more about what people are looking for, etc.
Yes, Amazon and B&N.com will sell your book if you self publish, but it will not be on any bookstore shelf. Anywhere. Bookstores do not buy self-published books. Period. The publishing company has no interest in selling your book--you've already paid them.
Published author; Professional Editor