Option 1:
They will have been planting the saplings before we reach home
This suggests the planting activity has started, but doesn't sounds right to me unless you qualify how long it's been going:
They will have been planting the saplings for 4 hours before we reach home
They will have been planting the saplings for 4 hours by the time we reach home
Option 2:
They will be planting the saplings before we reach home
This suggests much the same as 1 but without the need (in my mind) to stipulate how long it has been going on for. Simply, the activity has started and will be ongoing by the time we get there
As it doesn't need any edits to make sense, this could be considered as the correct option, but might depend on context - a more natural way to say this would be:
They will have started planting the saplings before we reach home
Option 3:
They planted the saplings before we reach home
This mixes tenses in the wrong way - it states something has definitely happened but gives a future time that it would depend on, so to be correct:
They planted the saplings before we reached home
Option 4:
They will have planted the saplings before we reach home
This option theorises that by the time they get home, all the saplings will have been planted.
(All the cakes will be eaten before we get to the party)
This option also seems correct to me
Summary:
I believe either option 2 or 4 could be considered correct, but would depend on what you actually wanted to say. The question is "Will the planting be ongoing[2] or finished[4] by the time you get home?"
To me, Option 4 sounds the most natural out of all of them.