You should dose for fluke every 10 weeks through the risk period - usually October through February / March up here, but at the moment is pretty much year round. Your vet will advise on the appropriate routine dosing regime where you are.
Dose rate for Fasinex 100 is 1ml /10kg, for Triclafas is 2ml / 10kg. There is a Fasinex 5% which has the same dose rate as the Triclafas.
Once opened, the Norbrook product (Triclafas) must be used within 28 days - which, given that it's a "don't repeat within 7 weeks" dosing regime, means you dose once and discard the rest. Even if you only dose 3 times a year, at £14 / bottle, that'll cost you £42 per annum.
The Novartis product (Fasinex) will last 12 months after opening if kept in the original packaging, properly closed, in a cool dry place away from light - so one container will enable to you to dose every 10 weeks throughout the year if your vet advises you to do so.
Triclafas min quantity is listed as 1L. Fasinex 5% and 10% both list min quantity as 0.8L.
I haven't found Fasinex 100 or 5% online in 0.8L packs, but the 2.2L pack of the 5% seems to be around £44.
Our this-year's commercial lambs are currently weighing 45-65kgs depending on when they were born. Our Texel x and Charollais x adult ewes weigh around 80kgs when fully fit. If you can't find some way of weighing yours accurately, go along to your local auction mart on primestock day, look at and feel the lambs and see what weights are coming up on the board to get your eye in, then go home and make an educated guess. Dose for the maximum weight you think they might be - you have to overdose by 10x or more the recommended dose to cause any symptoms, and should always dose the group to the heaviest sheep in the group.
Sight unseen and not being any kind of a Zwartbles expert, I'd guess yours would want to be dosed as though they weigh 60kgs if born early in the year and very fit. As they mature their weight will go up to (according to the breed info) around 85kgs - they probably won't hit that weight until they're in their third summer (2-shear.)
So, if my 60kgs seems about right, you'll dose at 12ml of 5% (or 6ml of 100) per sheep this time, gradually increasing to 17ml of 5% (or 8.5ml of 100) for the original sheep over the next 18 months.
If your vet says dose every 10 weeks year-round, then you'll use around 250ml of the 5% on these 4 sheep over the 12 months for which the Fasinex product would remain viable.
Hopefully I've highlighted the info you need and you can do the maths yourself once you know what the vet recommends as far as routine dosing goes. If they say you should only dose once and thereafter only if you get symptoms, then you could try to get away with the Norbrook £14 pack just once. Otherwise it comes down to which is the smallest cheapest bottle of Fasinex you can get - any of them will last you 12 months