The 100-400 costs about £1200 retail here in the UK, the 400mm f5.6 prime is about £1100, so given that the currently 12 year old 400mm f4 with IS costs about £5.6k, it's a fair guess that a circa 2014 400mm f4 prime, with IS would retail for probably in the order of £8k, so add the 200-400 zoom functionality, maybe £9k if you lucky, more likely £10k as the 200-400mm 1.4x is retailing at £12k in my local shop !
I'd say more likely (than a f4 200-400) is a 200-400 f4-f5.6 with hybrid IS and weather sealing at £3k would be a realistic alternative replacement to fit with the 70-200mm range, but wouldn't at all be surprised with a pump 100-400 f4.5-f5.6 with the new look white body, improved optics and plain IS, but then as argued before - isn't the 70-300L the logical replacement for the 100-400 giving the higher mpx options available now.