Mobile World Congress 2015 begins in March where Samsung has already sent out invites to what's almost certainly going to be its Galaxy S6 launch and HTC is expected to reveal its One M9. LG is, apparently, waiting until this all dies down before launching its G4 in April.
LG released its G3 in May last year. An LG source said: "Samsung usually releases its new flagship model in the first quarter of each year, and Apple in the second quarter. We need some in-between time to make a bigger splash." So it's not a surprise that LG is, according to the Korea Times, waiting until April for its G4 release.
HTC is also expected to unveil its new flagship handset, the One M9, at Mobile World Congress which starts 2 March.
LG is in no rush to release its G4 at the same time as the competition as it wants perfection of the final product to dictate release date. An LG exec said: "We rolled out the G3 in May last year so we do not plan to unveil the G4 at this year's Mobile World Congress and [will] spend more time perfecting the new phone."
Expect to see the Samsung Galaxy S6 and S6 Edge as well as the HTC One M9 unveiled at MWC in March, but based on this information we're not holding out hope for the LG G4 until April.