I want to purchase a DL4 to accommodate my DD-20 (I love layering delays). I know it's possible to use one exterior tap switch to link a DD-20to another BOSS pedal via one 1/2" Y-cable. Can this be done with a DL4?
I believe (Someone correct me if I'm wrong) a DL4 has to be modded to have an input for an external tap tempo switch, it just has 2 inputs, 2 outputs, and an input for an expression pedal.

On a side note, if you like layering delays, I would honestly recommended getting a Line 6 M9. It does everything the DL4 does, plus a world of other things.
Telecasters-->Dunlop Volume Pedal-T1M Pearl-AMT Japanese Girl Wah-Line 6 M9-Ibanez DE7-EHX Cathedral-->Mesa Boogie Nomad 4x10 combo