At 4iu, you can take it all at once. If your goal is fat loss though, I would suggest splitting it into smaller doses. 2iu in the morning and 2iu in the evening before training, had been my standard breakdown, but i have gone as far as splitting 4-5iu into 3 doses a day.
As for 50mcg T3 and muscle loss, it's going to depend on a few factors. If you are super lean and eating low calories to loose more fat, there is going to be more risk than if your food intake is high. Gh will help to retain muscle though and I find that GH helps maintain fullness while using T3. 50mcg is a fairly moderate dose though and you should be ok