* AutoZone up early, results show profit growth
* Carnival slides, hurt by downgrades, forecast of loss
* Futures: Dow up 3 pts, S&P down 3 pts, Nasdaq down 6 pts
NEW YORK, Sept 25 (Reuters) - U.S. stock index futures were little changed on Wednesday, following four days of losses on the S&P 500 and Dow, as investors grew increasingly concerned about budget and debt negotiations in Washington.
* Concerns over a potential U.S. government shutdown, and mixed signals on the immediate future of the monetary policy that has given support to equities, kept investors on edge.
* The possibility of a last-minute deal in Congress, however, eased the selling pressure brought by the uncertainty, analysts said.
* The Commerce Department releases August durable goods orders at 8:30 a.m. EDT (1230 GMT). Economists in a Reuters survey expect an unchanged reading for August orders after a 7.4 percent drop in July.
* New home sales data for August is due at 10:00 a.m. (1400 GMT).
* S&P 500 futures fell 3 points and were slightly lower in terms of fair value, a formula that evaluates pricing by taking into account interest rates, dividends and time to expiration on the contract. Dow Jones industrial average futures rose 3 points, and Nasdaq 100 futures lost 6 points.
* AutoZone shares rose 2 percent in light premarket trading after the largest U.S. auto parts retailer reported a 15 percent rise in quarterly profit, mainly due to higher margins.
* Carnival Corp shares fell 5 percent in premarket trading following various brokerage downgrades after the company warned it could report an adjusted loss for the current quarter.