Decimation of signal. How to plot frequency spectrum
The root of your problem is that you are not familiar with the Nyquist Sampling Theorem.

First of all, I question your premise that your sample rate is 70 MHz and that you can represent your signals, which range from 0 to 70 MHz, at this rate.  The Nyquist sampling theorem indicates that you can only represent signals up to half the sample rate, which is 35 MHz in your case.

When you downsample by a factor of four you are reducing your sample rate by a factor of four, which in turn reduces the frequencies that you can represent by a factor of four.  You are thus aliasing your signals, causing them to all be at the same frequency.  Thus, the one signal that you see after you downsample is really all four signals packed into one slot.