if you can connect the existing cable to the old light to one of the new lights and then daisy chain to the other three lights then you dont actually need the splitters BUT if you connect the existing cable to a splitter then you will only get three outputs to go to the other lights, however as...
yes but you will need two as the "4" way splitter only has three "outputs" so one splitter for two lights and a feed to another splitter for the other two
I have had a sensor I bought from Lidl outside my house working faultlessly for years and I have wired it as bernard suggested such that an indoor switch can be used to override the sensor and bring the light (4 of them) on continously
There was thread about this sort of thing some time ago.
Thic company does free samples of split glands..
https://www.icotek.com/en-gb/news-service/sample-kits#Split%20cable%20glands
All of that wiring should easily fit inside the new fitting. Just needs neatening up and maybe some shortening. Where is the cable entrance point on the fitting?
I also used v-tac tubes above a suspended ceiling and am very pleased so unless you have a duff one I can't understand why you think it's dingy.
Edit... Is the ceiling painted white or just plaster coloured? It makes a big difference.
where ever you hide the driver make sure you can access it easily in case it needs replaced.
you dont say how long the shelves are, are you going to have one supply for all three shelves ore a seperate driver for each shelf?