The phrase "four decades old" is correct and can be used in written English.
You can use it when referring to something that has been around for forty years. For example: "The hotel we stayed at last weekend is four decades old."
Now that stagnation is four decades old.
The internet, after all, is more than four decades old.
But the defense noted that the survivors were relying on memories four decades old.
The city was only four decades old but had already become the largest in the world.
Many of these pieces, some four decades old, frankly address homosexuality.
All five of the best-loved Christmas songs are more than four decades old.
This is the sort of work that film festivals are made for, in the interest of uncovering gems--whether new or four decades old--that we wouldn't normally see.
A. Most importantly we agreed on ideas, that the Democratic party had to move back to the center and had to express new ideas instead of following policies that were two, three and four decades old and no longer working either for our party or for the country.
The policy was a radical departure from the agency's more than four decades-old practice of allowing companies and individuals to settle cases without admitting or denying the allegations against them.
The world's largest and most sensitive cosmic ray telescope located in Ooty is made up of four-decades old recycled zinc-coated steel pipes.
If Trump can do it, out of the blue, without a solid ground game, and in a majoritarian system, then surely Marine Le Pen can do it, supported by a four-decades old, well-organised political party?
