Distributing a 40MHz clock to several PCBs
Rather than connect 4 buffers to a single input net, or drive multiple lines with a single buffer, you can use a fanout buffer IC.

For example, MC100EP14 is a 1:5 fanout buffer with 35 ps maximum skew between the outputs of an individual chip. The drift of this skew is not explicitly specified, but you can guess at it from the other specs. Typical propagation delay increases by 5 ps when the operating temperature changes from 25 to 85 C. If you imagine that the individual output buffers on the chip vary by +/- 5 C relative to each other, you can estimate that the skew will drift by 0.5 ps or less due to thermal effects, which are likely to be the dominant issue.