Welcome!
First thing in addition you'll need is a UVB bulb. You can get compact fluorescents that fit in a regular light socket, or you can get the tube kind that go in aquarium hoods.
Secondly, the 28 litre is about 7 gallons, so it's already too small for your RES.
A rule of thumb is 10 gallons per inch of turtle (38 litres per inch, or approx 15 litres/cm). If it's possible, take back the 28 litre and buy the biggest you can afford. If you can't take it back, that's OK for a short while, but start looking for something bigger. When your RES is full grown, s/he'll need a tank thats 70-120 gallons (265-450 l). It's better to start big, instead of buying multiple complete setups along the way!
Finally, the 4" rule is just in the United States. I'm not familiar with the laws in Ireland, but I know there are a few other members from Ireland.
Read through the main site (redearslider.com) and feel free to post any questions you have
