The Panasonic PJs have never really bowled us over, deposited getting rave reviews. It isn't normal for PJs to not be impressive!
Rather than putting the money into a 4K TV - which won't really benefit all that much over a good 1080p one at normal sizes - I'd out it into a good quality PJ. Have a look at the Sony HW40 or HW55. These might be 1080p projectors, but will look stunning on big screens, particularly the 55. Sony are about too release the VW300ES - their new 4K PJ which will be £5850, currently their cheapest 4K projector. If that is too much, 4L PJs might be down to around the £4k mark about this time next year.
I just don't feel that TV needs 4K. Yes, it is more detailed, but there's more than enough detail in a good quality 1080p one, particularly if it is being used for terrestrial broadcasts, Sky, Netflix streaming etc. I'm still using a Mitsubishi PJ from about 4 years ago, and there's no way I'd replace it with any TV, not even a 4K one. I use a 39" 1080p Pana TV for viewing everything other than films.