ARM could have done quad core at 350 nm with ARM7 cores if they had wanted to.
Looks like the hard macro size for a dual core performance optimized A9 core on 40nm is 6.7mm2, which allows 2GHz operation @ 1.9W according to ARM. So it would be around 13.5mm2 for quad core.
Of course, it only offers 1600 DMIPs @ 1GHz, compared to almost 5k per core for the A9, but die size does come with tradeoffs. If you could gang together 4 cores, it would be somewhat larger than 2.12mm2. Call it 2.3mm2.
2.3 * (350/40)^2 = 176 mm2 (still not counting I/O or other units)
A quad core A5 on 350nm would be a bit more modest, but I bet it would clock under 50MHz, have the performance of a 386, and dissipate at least as much power.