Luggage and bags:
1 very small, comfortable bagpack
Clothing/Shoes/Weather Gear: For hiking between monastaries: appropriate gear to wear for four days:
- Sneaker or hiking boots. There are snakes (I saw 3 in four days) and there might be scorpions (didn't see any).
- zip-up light-weight hiking-trousers
- function T-Shirt
- socks, underwear
I suggest you wear the above on your body, and pack only in addition:
- 1 pair of socks and underwear
- 1 function T-Shirt
- rain coat
Toiletries and Medical Supplies: very small pack, only essential basic things.
Photo Equipment: - as much as you want to carry and need
Camping/Beach/Outdoor Gear: - sleeping bag only if you plan to stay overnight on the peak of Mt. Athos. It might get cold, down to around 4°C even during the summer I heard, but they are supposed to even have some kind of bedding (to lie on, no sleeping bags) up there.
Miscellaneous: - some food and water. Occasionally, depending on region and season, there are fountains to refill water.
Take as little as possible. Almost everybody comes with a small (15/20 liters?) backpack that is half empty. No need to dress up, there are no women around anyways ;-). You don't need much for four days. Even if you stay an additional 4 days on request, you won't need much more. Wash once instead. It makes it so much more enjoyable to travel real light there!