$4K = too low. Probably by low-paid workers and doing plumbing and electrical themselves with no permit/inspection.
If this is a quick lipstick job to sell your house then no big deal for you (unless the buyer/inspector picks up on the cheap work). If this is for you to live in for a while and enjoy, then I have the following advice:
If you don't know how drains/vents work and should be constructed, then it's worth paying a real plumber to do the job.
At a minimum, the electrical should be done by an electrical contractor with an ESA permit and inspection. Don't take short cuts with electricity and water... If you do, at least test the GFCI operation and ensure you have a working smoke detector...
Showers need to be waterproofed. I'll assume you're doing an acrylic base, but if you do tiles on the wall and not the matching acrylic wall panels, then don't accept tiles installed on green drywall. Water moves through the grout and will degrade the drywall (even if it's green) over time leading to mould and whatnot. Cement board with either a surface waterproofing or underlying water/vapour barrier or using the schulter products are appropriate under tile walls.
Bathrooms need to be vented - don't forget the fart fan. Worth paying extra for the Panasonic fan - you can get it at a good price from the electrical wholesalers (your electrical contractor would know this). Figuring out the vent path to the outside may be easy or may be a challenge - there are rules for cutting holes in joists...
If you are going to be using this bathroom yourself, then it's worth spending a hundred or two extra on a better toilet. Amortized over a decade or two, this is pennies extra for getting a good flush everytime. I can personally recommend the American Standard Cossette (rona - $369) as I just installed one in my basement.
