When I connect all the pins to and I'm trying to add 2 binary numbers together it's just displaying the carryout pin as high when it should display 0100 on the output leds

 

It is the cd74hc258e

 

When I connect all the pins to and I'm trying to add 2 binary numbers together it's just displaying the carryout pin as high when it should display 0100 on the output leds

Post your schematic.

 

The scematic

 

I'm trying to add 2 binary numbers together

It is the cd74hc258e

What is the function of 74HC258? Is that the function you need?

This is the TTL part pinout, but it's the same for HC:

 

Hello,

As @dl324 said, lookup the function of the 74258.
You can find it in the attached PDF.

Bertus

 

I want to make a 4bit adder like the one in this video but when I connect it the output doesn't display what it is supposed to do

 

Hello,

In the video a 74283 is used and not the 74258.
That chip is also in the PDF I posted.


Bertus

 

I want to make a 4bit adder like the one in this video but when I connect it the output doesn't display what it is supposed to do

How can you add with a multiplexer?

Also, it's unwise to load outputs the way you've done.

 

Should I use transistors as switches and connect it to the main power

 

Should I use transistors as switches and connect it to the main power

You could use BJTs, MOSFETs, buffers (e.g. CD4049, CD4050), etc. If you don't intend to use the adder outputs to drive other logic, you can do what you're doing, but you're never going to get 10's of mA with VCC=5V unless you use 74AC.