If you go this route you will not only want to block the VHF from one antenna but block UHF from the other.
The good news is there is a device for that.
Weatherproof MATV Antenna Combiner/Joiner/Separator, Combines Full Band UHF/VHF Antennas https://smile.amazon.com/dp/B0744NTJX8/ref=cm_sw_r_cp_api_i_33VSKQHD5VZQNHEE5RTC
Just keep in mind that there will be some additional insertion loss incurred over a standard splitter/combiner due to the low-pass/high-pass filters.
There are many variations of the DB4, so here are two options depending on which one you have.
IF your antenna has a 300 ohm feedpoint, and uses an external balun (two pigtails going into a cylinder with a coax connector at the bottom) then connect the folded dipole onto the same two screws as the balun on the antenna and insulate the top of the folded dipole from shorting out on the db4's vertical boom.
If your DB4 has a box on it with a direct connection for coax, then you need a two way combiner, two short pieces of 75 ohm coax. Run one short piece of coax from the DB4 to the combiner, and one short piece of coax from the folded dipole to the combiner, and then run the coax from the combiner to the TV. Here is a LINK to a combiner. It would also require you to figure out how to mount the folded dipole (you should just be able to hang it under the DB 4's reflector with zip ties about 6 inches below the reflector (not to close).