if you are using a lager yeast then they typically need to ferment at much cooler temperatures (around 11-14 celsius). Keep in mind also that the fermentation process will raise the temperature of the beer up to about 10 degrees ferenheit above the ambient temperature of your flat. Fermenting higher than the ideal temperature range of the brew leads to much higher chances of 'off flavors'. While the advice given to you to let it ferment longer than 4 days was indeed sound, you may be disappointed with the final outcome of this brew if you did indeed use a proper lager yeast. On the other hand, if this was a brewing 'kit' that you bought, they may well have included an ale yeast in which case your temperature is likely fine (again, google its specs if you are able to identify the yeast you used). When all's said and done though, your beer has likely completed primary fermentation and all you can do is plug forward.
The hydrometer is very simple to use. There are many short tutorials on youtube that will show you how to do it. Ideally you want to have a reading taken before you pitch your yeast (giving you a reading called your "original gravity", or OG for short). Then when you suspect the beer is done fermenting, you take a second reading (called "final gravity, or FG for short). Many people recommend that you take several FG readings a few days apart to make sure they are the same (if the number is still lowering then you can see that the beer is still fermenting and not yet ready to bottle). If the two FG readings taken a few days apart are the same and within the ballpark suggested by the recipe then it's ready to bottle. You can also use the OG and FG reading to calculate the alcohol % of the beer (hence why it's useful, but not crucial, to have an OG reading as well). There are online calculators that will allow you to plug in the OG and FG readings and it will tell you the percentage.
For future brews, look up the specs on the yeast you use (google it) and ferment at that temperature for best results. Post fermentation of lagers typically need a "lagering" stage - a period of many weeks where the fermenter (or secondary) is cold crashed at temps in the 30s or 40s (ferenheit). I've only done one lager, which I did via this method, and tastings before it was really done had strong flavors of green apples. Once it was done, of course, the green apple flavor was no longer present.
I'm assuming you don't have a fermentation chamber or an easy means of proper lagering. What I recommend you do, for future batches, is to brew styles of beer (such as ales) and use yeasts to which your flat's temperatures are better suited. You can brew a lot of excellent beers at the flat temp you have, so there's no need to rush out and buy loads of specialty equipment.
