The new number system was introduced partly to deal with grade inflation (huge numbers getting A and A* and no way to differentiate between those that were really good and jus reasonable) and also as part of the raising standards across the school curriculum.
If you remember, SATs at KS2 have also changed and the curriculum made harder.
So a 5 which was a high C was what was considered to now be a pass - as part of raising standards, what would have been a scraped C now wouldn't be good enough for the new, slightly higher expected standard. That's what it was all about and not about establishing a new system with exact parity to the old one and not with a 'pass' being at the same level as the old system but slightly higher.
Why has a 4 been accepted by many colleges and used in some league tables - because although the government wanted to raise standards and the specifications became harder, not enough kids achieve the 5, as can be seen in those results tables. Standards have not been raised enough and to give all those who are not a 5 standard a 5 would generate grade inflation through the system.
So most schools report on both 4 as a pass and 5 and you can then see the difference. What you then see is that some schools have an awful lot getting 4s, rather than many getting higher grades - and people should be able to see that.
What about a 4? To be honest, the standard to get one is low. Some of those on a low 4 wouldn't have got a C in old GCSE. Colleges might take it because they want bums on seats for income reasons and it might be suitable for some lower level courses but couldn't be considered acceptable for A Level or equivalent. And as for Universities, they are running academic courses (even of varying difficulty/standard) so a 4 really isn't a useful grade for that level study.
What does a 4 show? It shows a certain level of competence, but not quite that to be considered what you'd hope students to achieve - so it isn't in those league tables. And yes, that does make lots of schools look likes poor performers, but if they aren't getting higher proportions to 5 and above, they are poorer performers, isn't it simply a fact. There might be all kinds of reasons why that is, in terms of catchment and starting point etc and value added measures like Progress 8 should show that, but in terms of raw attainment, some schools deliver less than others and yes, lots don't deliver well in absolute terms. Making the tables reflect 4+ won't change that but just hide some of the issues and how will that help anyone?
I appreciate not everyone can get a 5 (or a 4) but the tables aren't to make schools or local areas all feel good about themselves but to reflect actual attainment and give accurate information. Look on school websites to see the nos getting 4+ if you want to know about that measure. It's useful for anyone compared to the 5+ one.