This mod addresses the slop in Taz 4 (or Taz 5, 3, etc.) bearings and extruder holders.
The slop translates into uneven surfaces which are readily apparent on large flat horizontal surfaces, but also affect every other print in some way.
There are a few mods here and you can try all or one at a time.
First off, the bearing holders on the X carriage. You will notice that on a Taz 6 that Lulzbot started using bearing holder without a break in the circumference. There are two replacements here that fit existing inserts. Also, the bearings are changed out for the precision versions of the 10mm Igus bearings. I used the RJ4JP-01-10 version. Lulzbot uses a different precision bearing version on the bed of the Taz 6 which will also work. The 'P' in the part number is for 'precision' which has a tighter tolerance than the ones they used on the Taz 4 and 5 stock. I got mine on eBay for a decent price.
This mod alone will noticeably help with the X carriage slop.
The bearings can be difficult to install. I used a small vice for the single bearing with rubber protectors and a 7/16 long socket and a rawhide mallet for the double bearings. Do not use metal hammers and un-faced vices. The Igus bearing are soft and can be damaged if not handled properly. Slow and steady and light tapping only! If you need to get the bearings out of the double holder, cut the holder in half in the middle first. The single bearing can be a bit difficult without the right tools. You can remove some of the retainer ring at the insertion end to make it easier to install the bearing. It really only takes 1/4 to 1/2 of the ring to retain the bearing so you can use and X-Acto knife to cut some of the locking ring away to make the insertion easier. However the bearing will press into the one without modification if you just take it slow and use a c-clamp or small vice with rubber protection, or a long 7/16 (10mm) socket for a driver with a light mallet over a hole or bridged in a vice.
The next part is to stiffen up the extruder holder. if you grab the stock holder or large gear you will see that it moves around too much without much effort. The stiffened holder will stop that. You will need to replace the existing screw with an m3 x 30mm screw, using the old washer. This design makes it easier to install the nuts for the latch. You will have to use some supports on the bearing socket that faces the printer bed. I used sight support inside the nut chambers and struggled a bit with getting the support out through the small holes.
If I were to do it again, I would forego the supports in those slots and chambers. Make sure not to over-tighten the 3/8 lock nut on reassembly and remember the two washers outside of the rear bearing.
I also changed the bearings on the Y-axis bed, but did not use the newer bearing holders and only two of the precision bearings instead of all four because I could not get the bed to slide easily with all of those thing changed. I suspect is was because the two 10mm rails were not parallel, and I did not try adjusting the frame. That may have allowed changing out all the bearings and holders. I just settled for using the stock single bearing holders and two of four precision bearings which improved the slop in the bed. Changing everything out without adjusting the frame was binding too much to be used.