After looking at 5 cards from the top of my deck, can I place 1 card on the top and 4 cards on the bottom?

No, you cannot. After deciding the order in which you want to place the 5 cards, you must choose whether to return all 5 cards to the top or bottom of the deck.

What happens according to this [On Play] effect if I have 4 or less cards in my deck?

You will be able to look at all of the 4 or less cards in your deck. While looking at your deck, the number of cards in your deck will not be considered 0.

[OP01-073_r1] - Donquixote Doflamingo - One Piece Card Game

[OP01-073_r1] - Donquixote Doflamingo

Donquixote Doflamingo One Piece TCG Card

Card Description

[Blocker] (After your opponent declares an attack, you may rest this card to make it the new target of the attack.)[On Play] Look at 5 cards from the top of your deck and place them at the top or bottom of the deck in any order.