A regular, clear, white pine 2 x 4 stud grade can support static load of about 450 pounds for a 4' span and about half that for the 8' span with the lowest fiber stress capacity of 900psi. per the safe load table in WSDD So, the short answer is, don't use the 2 x 4 for your swing, and I think a 2 x 12 would be your least 2x for the span you're