Looking at the Huddersfield-Ravensthorpe section, the route north from Huddersfield Station to around Red Doles Rd (53.662158, -1.771262;
https://goo.gl/maps/u3TWwbdEeH42) looks easy enough to return to 4-track. The bridge at Red Doles Rd, being a 2-track bridge, will probably need replacing. From there to Bradley Jct (53.676217, -1.740797;
https://goo.gl/maps/RtbCtpofj322), 4 tracks may be harder to squeeze in. However,
@Mr Mean notes that Deighton station should be relatively quick to demolish.
The single track chord from Bradley Jct to Bradley Wood Jct (53.690328, -1.748032;
https://goo.gl/maps/gpSdooZjuEA2) I'm unsure about; I suspect it may remain as a chord for the time being.
Meanwhile, Heaton Lodge is going to be very interesting indeed...
The section of line between Bradley Jct and Heaton Lodge Jcts that crosses both the Huddersfield Broad Canal & the River Colne (just before its confluence with the Calder) (53.679614, -1.732175;
https://goo.gl/maps/WhLjNASLVGp) will need both its overbridges replaced, as the aerial view looks like (to me) that they could only accept 3 tracks in their present state.
The diveunder, taking the Down Line under the 2-track line towards Brighouse, was originally double-track, but I suspect track lowering and/or complete rebuilding would be required to accommodate OLE. The diverging route from the Up junction towards Huddersfield looks to be able to accommodate a second track with no major issues.
East of there, the route to Mirfield looks simple enough to reinstall the 4th track with little difficulty. Again, as Mr Mean pointed out, the island platform layout poses a problem; I suspect the island may have to be trimmed back enough to allow the two lines on the southern side to be slewed northwards, creating space for a 4th track at the southern extremity with a second island platform between the southernmost existing track and the new track.
Finally, it should be straightforward to Ravensthorpe; I can only assume and hope that platforms will be added in on the southern pair of tracks.