I don't think you should sell Property 3 as it's a huge earner for you.
Mortgage monthly payment E300
That seems very low, but in any event, the interest is what matters, and it's only €250 a month.
At the moment, you are making a profit of around €7,000 a year.
If you sell it, you will lose this profit and have to pay, say €6,000 interest on the €150,000 you would need to borrow to pay off the shortfall. So selling it would leave you worse off by €13,000 a year.
Let's say you sell Property 4
You pay less CGT, and keep the CGT losses available on Properties 2&3 against number 4.
You lose your cheap tracker, but it's €125k compared to €300k.
Let's say you sell Property no 1, but keep all the others
At the moment you have unrealised losses on these of €125k. The properties are worth €400k. So the properties have to rise by 30% to wipe out the unrealised CGT losses.
You keep your really cheap trackers.
You will have €250k cash.
You will reduce your exposure to property.
You will reduce your borrowing and vulnerability to interest rate rises.
This amount of cash will probably let you get a cheaper LTV mortgage in the UK, so you gain there as well.
On reflection, with all the information now to hand, I would say that you should sell Property no 1 and hold onto the others.