*A Lounge Access
 

I will be flying from FRA to SEA in F/C on LH and then connecting (4 hours) to the short flight to YVR (the same day) which only offers Y service. Can I access a *A Allaince Lounge?

No. You can't. You need to have a status to get in.

Unfortunately, it looks like you need to departing in first class to use the lounge (unless you are *Gold).

Welcome to Flyertalk!

I do not know LH's practice, but some airlines will let you use their First class lounge with a F class arrival boarding card, even though your ongoing flight is no in F. A good example is Swiss. An arrival F boarding pass + ongoing C booking to continental Europe is good enough for admission into Zurich's F Class lounge.

LH uses the BA lounge in Seattle, so it is worth to ask LH whether they provide an arrival/ connecting benefit if your ongoing flight is LH codeshare.