Just replaced a printhead on a Datamax i4208. Printed a label and it is all black. Put the old printhead back on and it prints normal. What's up?

I need more information. What system are you using to drive the printer with? If it is a Windows computer it is almost certain to be a faulty/outdated driver.

You need to replace the motor and the PSU.

When printing on less than full width media, the printer has a printhead-leveling cam to adjust the right side of the printhead
for even pressure distribution. To adjust the leveling cam:
1. With media loaded in the printer, print a label (press the
FEED + CANCEL buttons simultaneously) and
examine it..
2. While observing the printed labels, loosen the
Thumbscrew and move it to the left most position. (see
Example 1, below).
3. Next, move the Thumbscrew to the right until the
printed labels contain a complete, even image (see
Example 2). Tighten the Thumbscrew.

In Seagull driver/Bartender software, Present Distance may be called Tear Off and can be adjusted after checking Post-Print actions in Document Properties, Stock tab. Standard setting is 0.40 inches.
On the same tab, right below that, is "Top Offset", and that adjustment may get you the lower Start of Print you want. Ok to try negative number here.

1. The printhead in your printer might be scratched (evidenced by thin white streaks through your print). This is common, and the most likely cause. Printhead must be replaced.
2. Material might be stuck on your printhead (usually evidenced by wide white streak vertically through print). Printing surface of printhead can be cleaned with rubbing alcohol and cotton ball.
3. The ribbon might be wrinkling due to uneven tension, producing intermittent diagonal streaks in your print. This is somewhat common, but doesn't fit your description as well as 1 & 2 above. Mechanical adjustment and/or proper media loading solves ribbon wrinkles.
4. Poor pressure and/or printhead alignment. Evidenced by grainy, light or no print on one side or the other of the media. This is somewhat common, but doesn't fit your description as well as 1 & 2 above. Mechanical adjustment solves printhead pressure and alignment issues.

1. Right side not printing. Rotate round green knob (possible black center) found on right side of printhead assembly clockwise (or if necessary, through entire range) printing at each stop to check, until printing is satisfactory. If not fixed, next step is to do "Burnline Adjustment" as described in Operator's Manual.
2. Left side not printing. Make sure that large green plastic printhead latch on right is pushed firmly toward back of printer while printhead is pressed down, to make positive lockdown of printhead. Try rotating round green knob described in (1) above through entire range, printing at each stop to check. If not fixed, next step is to do "Burnline Adjustment" as described in Operator's Manual.

This is due to print server problem, you should change the options on the driver.

position fault is normally a problem with either the formatting or the printer is not calibrated correctly.
Hold the feed button down until at least 3 labels pass by the sensor.
Also check to see the sensor is directly over the die cut on the label if more than 1 up label

You probably need to adjust the leveling cam if you have not done so already. This is the green thumbscrew near the print mechanism.