I use DT lines in 4 wts and below and WF in 5 weights and above. Unless you are using specialty WF lines like a bass taper with more of the weight forward in the taper, my understanding is that a standard WF taper is close to or identical to the DT taper. The difference is the running line at the back end of a WF line.
To confirm go to this Orvis line chart and compare the front taper length of a 5 wt. WF and DT line. Both have 66 inches and a tip length of 6 inches. They are identical. Compare the front taper of all the WF vs the DT lines and they will be the same.
Fly Line Taper Charts | Reference
So if you don't need the running line of the WF line for shooting long casts, get a DT line. I cut mine in half and use a loop to loop connection with running fly line and save the other half for later use. It saves money. If you are going to shoot a 4 wt on long casts, then get a wf 4 wt. I hardly fish a wt line at those distances so I us a DT for my 4 wt floating lines.
You can get level running fly line in many diameters. I use the .027 diameter. Pick up the one that fits your needs to make you own modified fly line. It is how we used to do it before WF lines were developed.
LandBigFish - Cortland 444SL Running Fly Lines