You don't have to relock your bootloader, just flash 4.4 via Flashtool v0.9.14. The step of relock and unlock the Bootlader after flash 4.4 have no benefits at all, just waste of time.

BTW, via Flashtool you can flash baseband only, if you want.
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My Evolution: Sony Xperia Z1 - UB + 4.3 Rooted + Deodexed, HTC HD2, HTC Diamond 2, HTC HD Blackstone, Nokia 7070 Prism, Qtek 9100, Nokia 3100, Motorola T720i, Nokia 3310, Sony Ericson PF 768 (Blue)

How Get Root on Sony Xperia Z1 with Locked Bootloader and the Latest Android 4.4