since you have the dl 4 in hand, i would say that its best to go thru every setting in there and see if is working for you to get the "reverb" sound you wanna.
End of the day, if cant get the sound you wanna, its prolly better to look to a dedicated reverb pedal to get the job done.
delay and reverb is pretty much different sonically, its hard to use one in place of the other. For slap back echo, it might still be able to fake it, but room reverb, imho, a reverb pedal will prolly get the job done faster
the low delay setting trick to get reverb, with different delay, be it digital, analog or vst, always sound to me in having that slight unatural dimension characteristic which doesnt sound like how a reverb should be, to me.
hehe, john mayer rather particular bout his sound aint it. The reverb on the song other than amp reverb, could also be from post production. I know you are not going for that reverb entirely, but for better result, imho, a dedicated reverb would be much easier and faster.