What to do with a 4* +7 Soren?

Make a different Soren 5 stars and merge that one into your +7 one, he'll get access to the 5 star skills and refinery

Wait for another Soren, make that one a 5*, teach him Wind’s Brand and then merge that one into your 4*+7. Unless you really want to have a 5* supermerged one, it’s not really worth promoting your current one, because a 4*+5 (and any higher than +5) is stronger than a 5*+0.
We don’t know anything about a banner yet, since the late March roadmap isn’t out yet, but I’m assuming there will be one.

Soren tends to prefer +spd over +atk. And his res is actually pretty good so dropping it is kinda meh, particularly when you consider it is his superbane. His higher res is part of what sets him apart from Nino, particularly with the way his new Wind Brand decreases foe's attack by 7.

That said his preference of spd over attack was back when you would just blade him if you wanted him to be as useful as he could be. These days with Wind Brand he can actually play bait. Owl effect + -7 Attack and the fact QR can negate his speed woes against faster foes means +ATK is much more valuable. Maybe even preferable. That said the Res bane is still not nice.

The others have already pointed out that 4*+7 is greater than a 5*+0 so I won't harp there.

i would want a spd+ hp- or def- soren. and personally up to a 5 star due to liking to have more gold in my barracks and having way to much silvers

I would go with promoting the next one (unless he has the +spd/-hp or def IV’s) to 5*, teaching him the Wind’s Brand, and THEN merging with your 4*. And if you want to jump that one to 5*, then that’s completely up to you.

Don't feel bad. I actually started merging up some 4* as well.
I realized this: I'm merging up my armors, and then it's my dragons' turn. That's a year and a half worth of feathers.
In a year and a half I can definitely restock on those 3-4*, meanwhile having those sweet 4* +10 units for AA RIGHT NOW.

In the case of Soren, I have a 4* +5 one, and I plan to promote another one to 5* and merge him into the 4* so he can get his prf weapon.

I did something similar with Cecilia.

My first team when I started playing was Horse Emblem, and when I realized it wasn't going to take me to T20 due to low scoring potential, I decided to merge down my 5* Cecilia into a 4* +10, just so she could be a bit better for PVE, and divert my feathers to my Armor Emblem.
I not only DON'T regret that one bit, I also plan on doing that with Reinhardt once I get another +atk one to make a 4* +10.