Curious about adding two AC signals that are out of phase.?

4 ms is equivalent to a full wave of 250 Hz

this would lead to destructive interference of signals at 125 Hz,

375 Hz and at all frequencies of 125 + n* 250 Hz

Similarly it would lead to maximae at n* 250 Hz.

This may be disconcerting.

So the difference in delay needs to be a lot less than 1ms for it to be negligible.

I am surprised that any amp would have such a high delay.

You cannot fix this by phase correction.

Only by introducing a similar delay in the other line can this effect be reduced.

Otherwise expect some "weird" sound.

But perhaps this is what you want. You had to have some reason for using the looper in the first place.

Without knowing the device can you use a looper that is set to NOT loop in the other line to introduce a similar amount of delay?

Unless you have some form of delay introduced in the other line to compensate then I don't think you will enjoy the result.

Various other factors needed such as Impedance of source, impedance of load for both paths, what point you would be inserting the compensating components, i.e.pre compensation or post compensation. Remember that the components are all going to have different effects at different frequencies. You may want to try just adding another active component such as an A to D converter and back again in the "real power" side to give equal phase relationships. Try it and see. You will have some effect, but it may be good, bad, or negligible depending on your ear and material. Don't worry about it till you try. There's an old electronics phrase that might give you some help. ELI the ICE man, means that in an inductor (L) Voltage (E) leads current (I) but in a capacitor (C), Current (I) leads voltage (E).