Well it seems that the Amityville house WASN'T destroyed at the end of part 3 after all, as we see 4 priests entering the infamous building to conduct a series of exorcism's and purge the evil from it, which the house doesn't like very much (cue lots of low-budget special effects).
One week later, the estate agent clears out all the belongings in a yard sale. One of the items being a rather "interesting" lamp that one elderly lady purchases for her sister (you can see where this is leading).
Meanwhile some 3000 miles away in California, Nancy Evans (Patty Duke) and her 3 kids have just moved into her mother Alice's house, after being recently widowed. What arrives soon after? You guessed it, that bloody lamp Alice's sister bought from the Amityville house.
No prizes for guessing what happens next, as the evil from the lamp enters into their house and all wierd things start to happen within. Amongst some of the more interesting scenes we see a repairman getting his arm mangled in the waste extractor, a chainsaw going out of control after her young son Brian (Aron Eisenberg) picks it up, and a poor plumber getting a face full of goo whilst being trapped under the floorboards.
Fortunately, one of the priests who performed the exorcism at the house finds out what's going on and rushes over to save the day. Based on the novel Amityville : The Evil Escapes by John Jones, this made-for-TV movie is overall a much better offering than the two previous entries and is certainly worth a watch as far as low-budget TV movies go, but be prepared for the corny ending which I thought was a bit of a "cat"-astrophe.