10/10 update:
rnc hid 3 more tardises today under the same rules

Tardis 5:
Prize: 10 diamond blocks
Winner: cepheid
Picture:

Tardis 6:
Prize: 10 diamond blocks
Winner: CovertRaw
Picture:

Tardis 7:
Additional hints: the closest one to spawn so far, and for the first time no ender pearls are needed
Prize: 15 diamond blocks
Winner: CovertRaw
Picture:

Thanks again to rnc for hiding the tardises, and for everyone that participated, I had a blast and finally found one for once :D