Which lenses do you mean? The ones with Power O.I.S? They are not parfocal.
tl;dr:
I sunk ~300 Eur on a b4 lens setup, which I rarely use because it is annoying to setup/transport and not very ergonomical to use.
Imo the "best" "cheap" option if you want to shoot long lens is to get an old b4 lens + adapters.
I have something similar to this:
https://www.youtube.com/watch?v=92ggbr004NUBut that setup "sucks" as well. Setting it up is a pain. I need to detach the eazyhandle clone because otherwise it is too much stress on the bayonet. Then mount the adapter + lens and get out the powerbank for the lens for being able to use the zoom rocker.
Mine is as heavy as another camcorder + it is incredibly hard to keep stable on the gh4 because you don't have any stabilization and I don't have any additional fixed contact points for keeping it stable. I am trying to find an G-Cup EVF Eyecup for the GH4 for an eternity now, to have a more camcorder-like way of shooting. At some point I might try something like this Walimex Pro viewfinder, but that would be another thing to carry around and setup. Keeping it stable might be easier on a gh5 though with its IBS. I think a good tripod/shoulder rig would help a lot, but than it would be even more annoying to set up every time + even more stuff to transport. It is definitely not a "get it out quick and shoot" type of setup.
If you spend some time on ebay, then it is still a kinda "cheap" option to try. I paid 100 Eur for the lens + ~60 for the lens adapter + ~50 for 5.5 x 2.5 to 12pin hirose adapter cable + ~90 for the power bank. So in sum ~300 Eur.