Try and apply but do not hold your breath. According SEM Guidelines Chapter 3.5.3.2 you should have had a permanent contract during your L years. Only then can L-years exceptionally count towards the 5 years period on B.
Means you have to wait another 3 years till you have 5 years of continuous B.
I can accept that the L years don't apply, but will that make my previous PhD years don't count (because it is disrupted)? If that's not the case, it make things a bit easier.
When you apply for C you must have 5 years of uninterrupted stay on B. So far you have accumulated 2 years, as your previous stay on B was interrupted by the permit L period. Hence you would need three more years on B if your application is denied.
If your total stay (on B and L) is 10 years or more, you can apply via the regular route. If your total stay is less than 10 years you can apply for the early C based on good integration. In both cases you need at least 5 uninterrupted years on B when you apply.
L permit can be counted towards obtaining the C permit if it was issued based on a permanent contract or if it is clear that the stay is permanent in nature. While the first scenario (permanent contract) is straightforward, the second one is more ambiguous and some may argue that it applies to the OP's case. I am in a similar situation, also working at ETH: 4 years on a postdoc B permit and 2 years on L permit, currently in the process of renewing it (have to get either B or nothing, since L is valid for 2 years only). It seems that only the Migration Office can provide a definitive answer as our situation is rather specific.