be honest with you i've never it tried it hat way but cant see it wouldn't work, give it a go it'll either work or it won't
Download SP3 and CU4 and unpack them to separate directories (C:\SP3 and C:\SP3CU4)
Install the latest support tools from the unpacked CU directory
from a command prompt in the SP3 directory run setup.exe and supply the SP and CU locations like so
setup /PCUSource=C:\SP3 /CUSource=C:\SP3CU4
Let me know how you get on, i'd be interested to hear
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"Ya can't make an omelette without breaking just a few eggs" 😉