with(dfr[dfr$var3 < 155,], plot(var1, var2))
should do the trick.
Edit regarding multiple conditions:
with(dfr[(dfr$var3 < 155) & (dfr$var4 > 27),], plot(var1, var2))
with(dfr[dfr$var3 < 155,], plot(var1, var2))
should do the trick.
Edit regarding multiple conditions:
with(dfr[(dfr$var3 < 155) & (dfr$var4 > 27),], plot(var1, var2))