Some more rebus puzzles

I have some more Rebus puzzles:

Seeing how the first lot were a bit easy I decided to make these a bit harder. Good luck!

HINT FOR 3:

It has something to do with the fact that they are numbers i.e letters wouldn't work

Hint for 3 and 4:

For 4, it has something to do with the positioning of a certain letter/letters. For 3, there is no summing involved and the underlining just under lines the last number

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Next lot: Even more rebus puzzles!

Answer

Community wiki:

The first one is

Walking on thin ice (walking = stick figure, on = position, thin ice = 'th' in 'ice')

The second one is

Sitting duck

The third one is

Last but not least

The fourth one is

High L and S = highlands (answer by OP)

The fifth one is

Back out

The sixth one is

Hole in one