One of the main problems with digiscoping is the lack of light reaching the digital camera's ccd. Lowering the shutter-speed will help to gain more light but at the obvious risk of blur at the subject end and the camera end. Increasing the camera's ISO settings often have a serious side-effect in producing noisy/grainy images.
Using a high power flash gun helps in many ways and not just when the light conditions are gloomy, on Sunny days a flash gun can help to remove shadows on the subjects.
Clearly an external flash-gun is needed for this, as the Nikon's own built-in flash is woefully short on power to help generate additional light at digiscoping distances.....the scope/adapter will block the internal flash anyway.
To attach a flash gun to the Nikon 990/995/4500 you need to purchase the SK-E900 external flash adapter kit or the AS10 hotshoe adapter. The SK-E900 includes a bracket and the AS10 unit and is the better buy.
Although Nikon strongly suggest that only Nikon flash guns are used, many other flash-guns will work....but you do need to be aware that some flashes use differing voltages and may cause problems. A Wein voltage protector is available to make sure that no damage is done to the camera.
Flash-gun power is rated by a GN number.....the details are not relevant to users of the Nikon 990/995/4500, but the higher the GN# the more powerful the flash will be at maximum power.
On it's own, even the most powerful flash-gun isn't going to give much illumination at normal digiscoping distances, so a device called a Fresnel screen needs to be fitted in front of the flash-gun. These are available under a variety of names (Xtenda flash, Tele-flash et al), they usually consist of a couple of plastic support arms held out from the flash-gun's head and hold a plastic screen about 9 inches in front of the main flash unit. The Plastic screen serves to concentrate/magnify the light into a narrower and more powerful beam of light that extends the useful distance of the flash by a considerable distance.
These Fresnel screens are very lightweight, fold flat for easy carrying and are relatively chep (GB£35.00)
Nikon SB24.Speedlight + Fresnel screen
I Need to work on a better support bracket
Flash Comparison @ 20yrds
Flash Comparison @ 50yrds
More Light & Faster Shutter-speed
Without Flash @ 20yrds
Ignore the photo's caption
Without Flash @ 40+yrds
Flash @ 40+yrds
The Eye-shine problem
One of the side-effects of flash is the eye-shine in the subject, animals being very prone to it and it usually takes the form of bright white eyes (against the famous Red-eye with humans). Moving the flashgun away from the lens is a good ploy, but you can't move it too far away or else you may as well have remote slave flashes. Using lower power on the flash helps to reduce the problem and you can get lucky and not have get any serious side-effects on some shots.
Although you can rectify the eye-shine in Photoshop, you cannot bring back the iris detail.