1998 4.0
I know it's part of the cooling/ heating but I need to replace it and don't know what to ask for.
Picture is passenger side near firewall.

 

Welcome to the forum.
Part your looking for is called a heater control valve. Check your local auto parts stores, Auto Zone, Advance, O'Reilly...all list it and either carry it or can get it.
Google it up...even Amazon carries it via 3rd party vendors.
2 different types, a 2 port and a 4 port....yours is a 4 port....make sure you get the right one.
Not expensive....around 25 bucks, even from Ford.

 

+1` ^^^

But just to clarify a bit

Google: 1998 ford ranger 4.0l heater by pass valve

Heater By pass valves all have 4 ports
Heat control would only have 2 ports, and never used on Rangers, that I can recall

This 4 port valve is only used to stop flow thru heater core in the Cab when MAX AC is selected
The heat temp is controlled by the Blend Door electric motor in the cab on 1995 and up Rangers

Easy enough to replace, but you can also eliminate it if you want, just use two hose connectors in place of the by pass valve

Also when ever you do work on the cooling system, reverse the hoses at the heater core on firewall, this will reverse the flow thru the heater core as you drive which helps to keep it cleaner, back flushes the core while you drive

 

When I googled it up on Ford Parts site, it showed the 2 types. Maybe the other 2 port is for a non ac Ranger. Anyway, his photo shows the 4 port .

 

That is odd, never ran across a Ranger that used a 2 port, but haven't seen all Rangers, lol
My 1994 has no valves, just straight hoses to firewall
My MAX AC just closes the fresh air vent

I think the by pass was added in 1995 and up

 

Weird ! Ford Parts Giant lists these 2 under 1998 heater control valve. No description as to what the 2 port is for, just listed along with the 4 port.
See what happens when your stuck at home, crummy weather....lots of time to google this stuff up .

 

If you used the 2 port model what you would notice when it was closed(had vacuum) is that your temp gauge would randomly go up to above 1/2 and then drop down under 1/2
That's because the heater hoses are the thermostat bypass, when thermostat is closed coolant flows thru heater core(hoses) and back to water pump

If valve was closed during warm up engine would warm up much faster, lol, less flow, after t-stat opens some flow thru rad and back to water pump occurs but heater hoses still need to have flow, that's the design so "heater" in cab still works, lol
But if valve was closed then engine heats up a bit more, which opens t-stat a bit more then it cools down a bit, and then heats up a bit, so gauge does that up and down thing, and its almost random

If you see that on your Ranger.................the heater core is clogged up, so you probably have noticed cab not getting quite as warm as a year ago

 

Rockauto shows both valves available, something about with or without engine oil cooler.

 

Well that clears it up now I can fix my water leak!
thx all!