TL;DR: Get some YN-560III or YN-560IV flashes and a YN-560-TX, if you're sure Yongnuo's what you want.
You don't need to mod; just avoid the RF-603 (Mark I) triggers
First off, posts that say you have to modify Yongnuo gear are probably old and only about the Yongnuo RF-603 (Mark I) radio triggers. They're transceivers. These triggers auto-sense whether it's on a camera to switch into transmitter mode, but they do so via a signal on one of the TTL pins. It works for Canon/Nikon, but for mirrorless mounts like Fuji X, micro four-thirds, or Sony NEX this signal is on a different pin or is placed where it doesn't hit the camera contact (i.e., with a Nikon unit, since Nikon's pins/contacts are placed differently than Canon/mft/Fuji). Without receiving this specific signal, the unit never goes into transmit mode, and is always a receiver. Another bad side effect is that the test button even on Canon and Nikon doesn't work in hand--it has to be on the camera hotshoe to get the trigger to switch into transmitter mode.
RF-603II and later triggers work on the MFT hotshoe
Yongnuo addressed this issue with the RF-603MkII triggers (and the later RF-605 triggers) by putting an OFF/TX/TRx button on the side of the unit, so you can explicitly put the unit into transmitter mode when you want to (this also moved the on/off function to a place where you can reach it when a flash is mounted on it). Upshot: it now works on mirrorless cameras and the test button works in hand. So if all you want to do is fire a flash remotely, and you have some Panasonic or Olympus flashes to use with your GH4 off-camera, the YN-622, RF-603II, or RF-605 triggers all work just fine as manual-only triggers (i.e., all they can do is tell the flashes to fire in sync). The RF-602 triggers also work, since they have separate Rx/Tx units so the autoswitching deal never comes in to play.
Note: Panasonic and Olympus micro four-thirds mirrorless and four-thirds dSLR cameras all share the same hotshoe protocol. TTL, HSS, RC--they're compatible across the two brands. As far as we know, Panasonic's making both the Panasonic and Olympus flashes, and each model on one side of the fence, has a near-identical sibling on the other side of the fence (e.g., the Olympus FL-600R and the Panasonic FL-360L are essentially the same unit with different badging).
See also: Is there a radio flash control system for Olympus that allows setting flash power from the camera?
Yongnuo's YN-560 Gear
But if you're planning to go all Yongnuo gear, then your best bet for mft, which will give you not just remote firing capability, but also manual power and zoom control from the camera, are the YN-560 Mk III and later flashes with a dedicated YN-560TX transmitter on the camera hotshoe. Be aware, there are limitations. These are manual only flashes. You do not get TTL. You do not get RC. You do not get FP (high-speed sync; i.e., you can't use shutter speeds faster than 1/250s without getting black bands at the top and/or bottom of the frame). All you get are sync, and remote power/zoom control of the remote flash. The MkIV can also be used as an on-camera master if you want to have fill from an on-camera flash. The MkIII is slave-only. The nice part is that the radio receiver comes built-in to the flash, so you don't have to remember to bring along triggers and extra batteries.
At this time, Yongnuo (and most of the 3rd-party flash world) does not make mft-compatible TTL triggering gear. You are stuck with manual only. The only TTL-capable remote radio triggers for flash that work with mft are the Aokatec AK-TTL.
While the YN Canon/Nikon YN-622 and RT triggering systems work from the mft hotshoe to remote control Canon/Nikon TTL gear, that's a lot of money to pay for function that won't be fully realized unless you've also got a Canon or Nikon camera, too.
Reasons NOT to get Yongnuo
Cheap flash gear is cheap for a reason. Component and build quality/consistency, copy consistency, QA testing, how convenient/reliable warranty repair is--these are all things that can add to the cost of a flash. Yongnuo has cut back on some of these to pass the savings on to you. In addition, they reverse engineer their gear, and they don't actually have a grand design for an overall system. Their manual 560/60x triggers aren't directly compatible with their TTL 622 triggers, or their reversed-engineered Canon RT system gear. Compare that to Phottix, RadioPopper, and PocketWizard systems, where there's a variety of TTL and manual triggers that work together. Consider where and how you may be planning on expanding your lighting system, as you would your camera system.
