The old adage of "If it sounds too good to be true, it is!" Think about it, why would any company, especially in this economy, send out $4000 to someone with whom they had no long term working relationship, in hopes that they would perform a task for them? The recipient could just as well walk away with the money and do nothing. Would they take that loss?
Mystery shopping generally pays $10-$15 for jobs performed. Of course there are some that are less and some that are more. Most jobs work out to less than or about minimum wage by the time you take into account studying what you are supposed to do, go do it and fill out the report. Most companies pay you 4-6 weeks after you perform the job. Mystery shops YOU have to request or self assign, they do not ever just send you a check and tell you to go do them.
Do not be lured into signing up for lots of 'special offers' in order to get mystery shops for '$500 gift cards' or 'free laptops' or similar stuff. They are just scams to get them referral fees for sending potential customers.
Do not ever pay to shop. There are many 'services' out there that will supposedly send you shops and the vast majority of them are sending lists of companies to sign up with or lists of available jobs--all of which are free here or elsewhere.
Some outfits now are offering to 'train' you for a fee. Training for jobs to be performed is available with many of the companies you would sign up with for free. No training overrides the specific instructions of the job you are to perform that are provided by the company from whom you accepted a job.
Look through this forum. There is a specific section for New Mystery Shoppers that includes a lot of the basic information about how this works and information to help you get organized so that your business can function smoothly. That is free because it is in the interest of all shoppers that their fellow shoppers be reliable, trustworthy and able to deal with shops in a businesslike manner.
In the Mystery Shop Company Discussion area, the very first thread includes at its beginning a list of more than 200 legitimate companies to sign up with. If and as you discover they are charging a fee, walk away. Everybody is trying to make a buck these days and charging shoppers to be able to shop or for 'training' or 'leads' IMHO is about as low as you can go.