That double-headed eagle is a good sign you have an Austrian coin, Jimi. And this one is big, bold, beautiful, and made of 0.443 troy ounces of gold. Nice! The value of gold changes every day. Be sure to look it up.
Even though the date says 1915, the coin wasn't struck that year. Austria has been turning these out with the 1915 date since they started, even up to the present. Just about all 4 ducats you see today (as well as 1 ducat) are dated 1915, but the pattern has been in use since Franz Joseph started his reign in 1848.
On this page we address Austrian 4 ducats from 1867 to 1915, and beyond. These 4 ducats measure about 40 mm in diameter; single, 1 ducat coins measure about 20 mm in diameter. Our page for Austrian 1 ducats in the same date range appears at this link. Austrian 4 and 1 ducat gold coins are complicated subjects, as there are many subtle variations. Don't invest large sums in them unless you are sure you know what you are doing.
Coins dated 1915 -- All 1915-dated coins are worth their weight in gold. These days, that is not too shabby. At the time of this writing, the US dollar is weak and gold sells for about $1000 US dollars per troy ounce. Base price, then, is 0.443 x $1000 = $443. Look up the current price of gold (it changes every day) at kitco.com. A dealer is usually involved when buying and selling gold coins. Figure typically a 10 to 20 percent commission for the dealer services, often more for small purchases of only a few coins, less for large purchases.
But watch out. There are coins which are very similar, but they are very thin and do not weigh as much as Jimi's coin. They have a base value worth about one-quarter of Jimi's. Most of them do not have the small 4 in parenthesis on the reverse. See the picture. They certainly weigh less than genuine 4 ducat coins which have a total coin weight of 13.96 grams.
Austrian ducats dated 1915 are restrike coins and are used today mostly for investment.
Since these are such popular coins, you can be sure crooks are in the act. The counterfeit piece shown with the black background contains no gold whatsoever.
The discussion above applies to 1915 restrikes. Next we address non-1915 Austrian 4 ducats.
Coins before 1915 -- Coins dated before 1915 are much more valuable because they carry numismatic (coin collector) value in addition to basic gold value. To obtain approximate value of these coins first compute the basic gold value outlined above and then add collector value. Collector value is a function of the condition, or amount of wear, of the coin. If the coin has severe problems, like holes, scratches, cleanings, or stains, the collector value vanishes and the coin is worth basic gold value only. Be sure to check our Terminology page to understand the many nuances of collector value.
Here are approximate collector values for Austrian gold 4 ducats before 1915. Add these to the basic gold value to obatin an approximate retail value. Dealers will buy coins at substantially less than retail. Figure at least 10 percent dealer commission when buying and selling gold coins, often more unless you are buying a lot of coins.
COINS DATED BEFORE 1872:
circulated: add $400 to basic gold value
uncirculated: add $2000
COINS DATED 1872: see below
COINS DATED 1873 TO 1877:
circulated: add $200 to basic gold value
uncirculated: add $1000
COINS DATED 1878 TO 1910:
circulated: add $100 to basic gold value
uncirculated: add $500
COINS DATED 1911 TO 1914:
circulated: add $50 to basic gold value
uncirculated: add $200
COINS DATED 1915: see discussion at the top of this page
There are minor variations in 1872-dated coins. Some are worth more than others. The variations involve the amount of hair in Franz Joseph's beard. Seek out a skilled dealer or collector if you have one of these. It takes personal examination of the coin to figure out if you have the valuable version or the more common version.