Warm glycol defrost

[sparkatan]

Which is the best method to retrofit a chilled glycol system evaporators with warm glycol defrost, 3 pipe or 4 pipe? Seems to me 3 pipe is fairly cheap to install and 4 pipe capital cost is more expensive, however, 3 pipe forces more warm glycol back into the cooling circuit.
What about control valves? 3 port on the 3 pipe flow only and on flow & return on the 4 pipe?
Not done this so welcome to any ideas.