Fencing wire's a bad choice -been there and tried it. Mind you the flash-bang-wallop was quite impressive but it made one hellava mess of the DB board. All very nicely plated in a mixture of steel and zinc.
Bear in mind that 4kW of lights will demand quite a bit more during the warm up period since the cold resistance is very much less than the hot.
Also trying to switch any form of hot-wire lamp on for only 1/2 second repeatidly is a sure way of keeping the lamp manufacturers in business.
If you only want a short duration, high intensity flash then go for a strobe type device (flash gun) which uses a gas discharge tube. These can produce very bright light and are designed for continuous flashing. Although the discharge is over in a matter of milliseconds, the persisence of the human eye makes it look much longer.