How much recording time on a 4xDVD-RW disc set at each speed ?

Answer 1

The 4x has nothing to do with recording time, it only denotes how fast you can read or record.

Recording time depends on the bit rate you use.
Most recorders specify only High Quality, Good Quality, etc.
They usually use 6 Mbits/s, 4 Mbits/s, or 2 Mbits/s (the more the better).

Say the DVD-RW is 5 GBytes
Then at 6 Mbits/s (ignoring audio which is usually 128-256 kbits/s) you can record

5000 * 8/6 = 6,666 seconds or about 111 minutes.
With audio, it is approx. about 1.5 hours.