As a general, rather crude, rule. we use "since + past time" - present perfect:
I have lived in Prague since 1998.
The past time can be indicated by a past-tense verb:
I have lived in Prague since I arrived here in 1998.
Many adventurers have traveled this way since the recent troubles began.
Now, on to your #1: It seems like forever since the four of us have been out to eat
We can look at the actual situation in two ways:
a. The four of us have not been out to eat for a very long time/since 2001/since forever. -Present Perfect.
b. We last went out to eat many years ago. - Past simple.
The speaker of your #1 has conflated these two idea in his/her mind, and produced the utterance with the present perfect. One would have expected:
It seems like (it has been) forever since (the past occasion on which) we went out to eat.
So, according to the crude rule I started with, your #1 is 'incorrect'. However, in real life, such conflation is not uncommon. I do not think anybody would consider it strange.
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