WILL THE SUNPAK FLASH 4000 AF FOR NIKON WORK ON MY NIKON D3000?

I need a cheap flash for a Wedding, the one that I use is in the shop

I'm not familiar with that flash. The things you want to find out is if it will do TTL or if it is strictly manual. A manual flash will work OK - that is what we old timers grew up using - but it does take some setting adjustments.

But the main problem is one of frying your camera.

Depending on the age of the flash, some older flashes had high-voltage present at the contact points as older cameras just had a set of dry contacts in them to trigger the flash. But modern DSLRs use electronic contacts in the camera and these are expecting to see low-voltages present from modern flashes.

So there's the potential to fry the camera by using a flash with high voltage present.

There are two things you can do.

1.ascertain that the Sunpak flash presents a low voltage, and confirm with Nikon that the voltage is within tolerance.

2.use a flash isolator, that fits on the hotshoe between the flash and camera. These gadgets isolate the flash trigger voltage from the camera and presents a safe voltage to the camera.

Wein-Products-W990560

SMDV-Hotshoe-Adapter-SM-512

Or just wing it and see if it will burn your camera out or not.

In manual mode, it will work. It will not work in any sort of TTL mode, this flash is much older than the Nikon iTTL flash system your camera uses.

If you can work with a manual flash, you are OK. If not, look at the Vivitar 285HV (about $90 new) which is a very good auto thyristor unit, which will work just fine with your camera.