If you plan on ordering a brand new dress through a salon, you are probably out of luck. You can inquire about rush orders, but this can be pricey. Start calling salons in your area and ask how long a rush order would take.
However, there are plenty of ways to get a nice gown within four months:
Start by going to a bridal salon and try things on, to see what looks nice on you. Explain your situation and perhaps they will have a sample gown that they can sell you. It might need a good cleaning and some alterations/repairs, but see what they have in your size that looks nice on you.
If you try on something that you like, hit the Internet and see if anyone is selling it, or something similar. Sometimes brides sell their gowns after they've worn them in the weddings ... and some girls buy a gown, then buy a new one and try to unload the (unworn) first gown. Try looking on eBay, Craigslist, preownedweddingdresses.com, oncewed.com and savethedress.com.
Also try on bridesmaids' dresses. Many are quite elaborate, and a lot of designers actually sell the exact same dress in white (for a higher price) as a bridal gown. Inquire about a rush order on bridesmaids' gowns, but those typically come in much faster than bridal gowns because there is less labor involved.
Nordstrom.com, dillards.com, Jcrew.com and AnnTaylor.com sell bridal gowns, so check out their websites.
Look in stores that sell formal dresses ... boutiques, department stores, prom stores like Windsor. See if you can find something in white/ivory.
There's no rule that you need to be in a white/ivory dress. If you like color, then get a colored evening gown or a colored bridesmaid dress. Look in the aforementioned stores, and also bluefly.com and overstock.com. (I saw several white evening gowns on Bluefly that cost under $500 and could EASILY pass as wedding gowns ... here's a gorgeous one: http://www.bluefly.com/Marc-Bouwer-GlamIt-white-silk-off-the-shoulder-draped-gown/cat20112/306354401/detail.fly)
A bonus about this is that you would have the chance to score a designer-name gown without breaking the bank ... for example, Bluefly.com has a bunch of colorful evening gowns by Badgley Mischka, Vera Wang and Nicole Miller, all for maybe under $600 apiece. Which is expensive for a regular evening gown, but a steal when it comes to wedding gowns. You would probably pay at least two or three times as much money for a "wedding gown" by the same designer.