On most PICs RA4 schmitt trigger inputs require a minimum input level of around 0.8 * Vdd to be recognized as a logic 1. Assuming you're running right at 5V, this would be around 4V.

The Linx ES series receiver data output level for a logic 1 is around Vcc - 1.1 min, Vcc - 1 typical, or Vcc - 0.9 max. You only have around 3.9V min to ~4.1V max. Best case is around .1V above the minimum level required by RA4, so this could be a problem.

A TTL input would have been nice, but you can always use an FET or transistor.