Ok, here's a quick way to draw perfectly straight horizontal lines:
Draw a vertical straight trendline first in an empty space on your chart. You will see that in MT4 that a vertical trendline when drawn tends to snap into place perfectly straight. Once drawn, open the line with left mouse click to show the 3 editing handles;
Hold down the [Shift] key, left mouse click and hold the bottom handle of the line then drag the selected handle to the right (or left) and immediately the trendline will jump to a horizontal position and perfectly straight. It can now be lengthened as required...make sure you use the [Shift] again when doing this so as to preserve line straightness;
At this point, left mouse select the line to open the editing handles, left mouse select the centre handle and slide the line where you want it at desired S&R positions. Make sure when moving the line around and placing it in position that neither line end handles come into close proximity (about 10 pixels) of either a high or low point of a bar or candlestick as the end of the trendline will be attracted to that high or low point which may result in distortion of your perfectly horizontal trendline (NOTE: This "magnetic" attraction of the line end to a bar or candlestick can be modified or switched off via Tools/Options/Objects/Magnetic Sensitivity).
This description may sound slightly convoluted but practice it a few times and it will be easy.
To assist in understanding the above process I have attached a short video.
Hope this procedure helps.