You don't need to push in the clutch to engage 4 hi. All you need to do is ease up on the gas pedal and shift firmly and gently into gear. I've shifted on the fly in all gears from a 5 mph to 65 mph pulling a loaded trailer in 6 inches of snow. The t-case is designed to be shifted on the fly into 4 hi. To shift into 4 low, you must be stopped, as you have to move the shifter through neutral to engage it. If you have automatic locking hubs, just shift and go. If you have manual hubs, they can be locked in, and you can be driving in 2 wheel drive, and shift on the fly when you want to.
It's about impossible to shift the tcase if you're at a complete stop though. When going into 4lo, be rolling, but just barely.
Is this the way the atlas tcase works? On any np case I've always been able to shift into 4hi or 4 low at a dead stop. In fact, to get into 4 low I thought it was recommended to be at a full stop.