No one can know, except for the one who made that track!
If the surround channels are identical, there are good chances that LFE was extracted from both left and right, so mixing it back to them *may* give you back the *original* 4.0 track.
BUT... I would leave the 5.1 as-is, to retain compatibility with more devices - there are some players/receivers that interprets 4.0 as L-R-Sl-Sr and not as L-C-R-S!
If the surround channels are identical, there are good chances that LFE was extracted from both left and right, so mixing it back to them *may* give you back the *original* 4.0 track.
BUT... I would leave the 5.1 as-is, to retain compatibility with more devices - there are some players/receivers that interprets 4.0 as L-R-Sl-Sr and not as L-C-R-S!
