I have a unlimited t-mobile sim and it's picking up 4G in my IPAD. I will use that until 3 start their 4G because like with 3G that's where it's going to be at
It's a false assumption that Three will offer the cheapest high use tariffs on 4G just because they did so on 3G. In the 3G marketplace, Three were the last to arrive and had to differentiate their offering somehow, especially with the '3G or bust' nature of their network - and the resulting iffy in-building coverage - in areas as they stopped their 2G roaming arrangements. In 4G, Three will either be the second to launch or will be launching with the rest, depending on what they do with the 1800MHz block they are buying from EE when they take control of it (EE are hanging on to it for as long as possible to preserve their 4G monopoly).
Unlimited use 4G is a particularly dangerous offering for the mobile networks. These days, it's trivial to configure a cheap smartphone to share its data connection over Wi-Fi, leaving terms and conditions restricting the use of unlimited data usage SIMs to mobile phones with no practical effect. If people have unlimited data in a 4G area, they may feel the connection is fast enough to get rid of their landline and associated fixed broadband. That leads to an impossible 'arms race' - it simply isn't economic or even possible within the limited amount of radio bandwidth available to provide enough wireless bandwidth to replace fixed broadband. Unlimited 4G data tariffs may also encourage further growth in data demand in non-4G areas, which the networks may be unable to service.
I suspect the way ahead will be tariffs that include high (by previous standards) but limited use. This will permit existing products with unlimited data to be precluded from the 4G network, which will help encourage migration away from these tariffs by the heaviest users. It was no accident that O2 abandoned unlimited data tariffs for phones as users upgraded - they couldn't keep up with demand. 4G offers the perfect carrot for the networks to tempt users off legacy tariffs they wish to withdraw.
A particular danger for the mobile networks of offering unlimited 4G data is that the ASA is taking an increasingly dim view of 'unlimited subject to fair use policy' - if you advertise something as unlimited, the customer increasingly expects it to be truly unlimited and may have a complaint for false advertising if there are undisclosed limits.
My mobile is on Vodafone Red SIM Only, which has truly unlimited voice for personal non-business use only. So long as your call pattern doesn't suggest intensive business use, there's nothing Vodafone can do to ask you to moderate your usage. The kind of abusive business usage Vodafone wish to prevent on a consumer tariff should be fairly obvious - anyone making 30+ short calls to strange numbers each day is clearly telemarketing, and anyone racking up 3000 or more minutes every month should expect questions unless the usage was predominantly long calls to a handful of numbers. Most people's personal usage will be repeated calls to a limited range of numbers with most of the rest being occasional short calls. Overall, I suspect the total additional call volume over the 1200 minute tariffs that Red replaced is rather modest, making the limited increase in costs worthwhile for the marketing benefit of being unlimited.
When I make voice calls, they tend to be fairly long - since switching to this mobile tariff, I use my mobile to make these calls so as not to risk the high costs of going over an hour on an 'unlimited, redial after 60 minutes' landline tariff.