I've actually seen an Apollo-era flight computer (sans diskey interface unfortunately) at VFC West a couple of years ago, the deal with that stuff was that they only used them for very specific tasks having to do with calculating numbers. They didn't have fancy inputs or outputs. They didn't have all that much in the way of storage at all. Part of what makes that 4K so special is that it was static, non-volatile, and hardened against outside interference. And all it was needed for were specific pre-programmed calculations based on current input. It didn't need to drive a 3D display or sing "Daisy" or any of those vital spacefaring tasks.