[mako1]

Joe makes some good points.It will look plain but to answer your question: poplar 1x4's are poplar 1X4's.No difference what you use them for.Is this going to be painted?Poplar takes primer and paint well.If staining there is a lot of color difference in poplar from the sap wood to the heartwood which you may want to consider.
If using for base you do not want a flat top on it.All it does is collect dust.You could route the edge with a profile or put a cap molding on top to help .
Poplar machine very easily.

 

[mako1]

Pic does not work.Maybe to few posts.

 

[mako1]

I like you're style kwickfishron