Since this is at the start of play, you can actually mess around a bit narativistically .
You officially start play, 4 years after that adventure.
Narrate that scene a bit,
and then do a flash-back to the adventure you wanted to do as an intro.
For example you narrating as the GM:
Our story starts with you all meeting up in a tavern -- a bunch of old friends who haven't seen each other in years.
[do some roleplay here]
You get to talking about the last time you were all together,
back before you all settled down and got real jobs
What crazy adventures you had,
and to think it all started in this very tavern, four years ago to the day.
You then tell the players that we are going to roleplay a flash-back.
And then you play that adventure.
You can do some roleplay in the tavern.
So they get to the feeling that these are their characters.
Then in the flash-back they are playing a younger version of themselves.
So you haven't skipped forward in time -- you are not taking away adventures they could have had together.
Because in this narrative it is already established that they have spent the previous 4 years "settled down".
You are instead giving an extra adventure, one that happens in the flash-back.
The other nice thing you can do,
is start/end each flash-back session, with a scene in the tavern.
Building up the frame of what will be your next adventure.
E.g. if it seems like they will be done with the adventure by the end of session 3 and your next story involves an army, then session 2 has some people spreading rumours about seized crops 2 counties over. and maybe session 3 has a recruiting sergeant turn-up and begin his speal;