Well, since we're thinking forward to 5th, I thought I would act on a vewy good suggestion from this last week and propose we have a good look at the Kustom Force Field option for Big Meks
Basically, instead of a Shokk Attack Gun, (because that's the only other thing worth buying for a mek) you can buy a Kustom Force Field. The KFF bestows a 5+ cover save on all units with models within 6" of the mek, and any vehicles within that range are treated as Obscured.
In 5th, Obscured vehicles get a cover save as well.
So imagine facing down a formidable Dred Mob, or a line of Battlewagons, all with a 5+ chance to negate any glancing or penetrating hit against them. Boyz dashing about in the open with invulnerable saves, etc.
Of course, in 5th independent characters can be targeted like anyone else unless they join a unit (and then they become part of the unit), so 'shoot him with railguns' is a real option. The best players will probably keep their Meks inside units of boyz or gretchen to give them a little protection.
So how do you deal with a bunch of ramshackle ork vehicles (or half-naked boyz) suddenly haivng invulnerable saves?