It really depens on the FX applied. My in the meantime aged 3930K with a Fury-X card does playback the 4K 30p files from my AX100 well at Best/Full, these are 100Mbps XAVC-S files. I mostly apply Vegas own plugins and that works fine, even with 6 and more. Boris Continum plugins are slowing things down quite a bit. Mercalli 4 stabalization even more so. 

You ahve to look at least at a good 6-core socket 2011-v3 system and SSDs. I good AMD card, 390X, 480X Fury-X is recommended; maybe an 8GB card would do better than my 4GB Fury-X but not sure. I would also go for about 16 to 32GB RAM.

On project media, Right click on the 4K media > Create Video Proxy - that can help faster the playback.

On project media, Right click on the 4K media > Create Video Proxy - that can help faster the playback.

"What i want .... with out pre rendering the clips"

Depends on what 6 effects you have applied. I am sure you know some take more horsepower than others, but 6

on 4K...asking a for a bunch of power, not sure you can get it, but again depends on what effects you are wanting to apply.

I use a combination of sony effects and magic bullet

On project media, Right click on the 4K media > Create Video Proxy - that can help faster the playback.

I couldn't find this, is it in pro 11 ?

Proxy generation was first in Vegas Pro 12.

Is it worth up grading to Pro 13 just for this ?

Maybe not just for proxy editing, but the totality of 13 was worth the upgrade for me. 

What else in 13 was worth the upgrade ?

I think your real issue is that you'll have to get past 11 to output 4K. 4k encoder options were added after 11. Frameserving to other applications may be an option but I didn't try it with 11.

Mike , magicbullet is most likely your  culprit , slows down my system bigtime.Sorry I dont know what system may play it full speed, mine is fairly powerful and it chokes ,especially if other filters along with mb.

Just checked my processor , it's a i7 2600K 4 core 3.4 ghz.  I like magic bullet and use it a lot with some sony plugins along with red giants  Cosmo plugin. The more advanced I get with editing the more effects I use. I get along now by pre rendering on the time line but it slows me down a lot.  I love shooting UHD 4K on my GH4 , it looks great, even though I out put 1080 or even 720. It just looks so good even when uploaded to YouTube. Check out this video I made two days ago.

The slomo was all gopro at 120 fps 720p.  I do a lot of gun videos but I also do a lot of modeling videos like this.

Modeling vid was done on a GH3.

I would like to find someone who has a system that does what I want without having to experiment . Whether it's a high end i7 machine or a HP xeon work station . That way i'll know what direction to take.

I'm learning a lot so I hope I keep getting some more good info Thanks

I built my current machine 2 years ago, which is a long time in computer terms, but it's served me well and a lot of the components and discussion here are still relevant: https://www.vegascreativesoftware.info/us/forum/new-x99-computer-for-editing-gh4-4k-video-in-vp13--98503/ I do cut GH4 footage on it mostly smoothly without proxies and with GPU acceleration off, but I don't stack lots of FX. Just a bit of colour correction. You could use that spec. as a start and update the components and see how the budget looks. e.g. Asus X99 Deluxe ii, perhaps SSD instead of HDD, a later i7 CPU etc.. If I was building now I would probably put in an ASUS RX480-8GD5 GPU, but I would do a bit more research first on whether an NVIDIA GTX 1080 might be worth the extra $.

You could wait a few days to see if VP14 changes the scene in terms of PC spec, in particular GPU acceleration, although I suspect that is not changing with this release.

hi nick, interested also - wondering whether 480 @ $400, or 1080 at $1k would see an equivalent boost in performance, ie, at least double ;-)? 

Just spotted this. Doesn't mean to say the figures translate relatively to VEGAS though. https://forum.blackmagicdesign.com/viewtopic.php?f=21&t=51840

yeah, been following the threads over there closely too. does seem that the 1080 has quite some bang for the bucks, but i'm not that convinced it's worth over double the cost of a 480.

am in no particular rush since i'm still working hd and unlikely to move to 4k until i see a decent camera at the right price point and a box that'll handle 4k with relative ease. my present 4770 / 32gb / gtx 960 works well enough for hd but wasn't too happy when i tried some test 4k stuff.

Since the GH4 will deliver UHD 30p or 4K DCI 24p only an 8core system like my i7 5960X works fine with the internal recorded GH4 footage. Overclocking the system helps for Vegas. To have full playback speed with filters like colour correction a GPU like the R9 390X 8 GB works fine here. Limitations are the number of filters, or the filter you use as said here. 

Another limitation will derive by the type of footage. FS7 XAVC I  UHD 50p will be tough even with this overclocked system and will not run at full fps. Here some tricks can help - set the project properties to UHD 25/30p or reduce the preview size to preview half. Also Cineform will not run with full speed. I cannot say anything yet about UHD ProRes. 

So even with high-end systems you will see some limitations. Limitations that derive by Vegas. Because for Edius, resolve or even the Catalyst Edit UHD 50p with XAVC I runs at 50fps with such a system.