I was driving just a little while ago and ive been noticing that it didnt really want to shift into 3rd from 4th like pulling a hill and now its doing it both in and out 4th.Could this be the accumulator or isnt there a sensor for every gear because its not pulling codes for that to be wrong.Theres been a recent service done on it and the fluid is full

Sounds like your TCC solenoid switch is out of adjustment. This write up is for 96-99 but should give you a good idea of where to look and what to do. https://www.gmforum.com/electrical-133/tcc-solenoid-switch-284972/

One common issue with the 4T65E is stripped splines on the 4th clutch hub. You could be intermittantly losing connection between the 4th hub and the overunning clutch. Are you sure it'* a 4th gear problem and not a TCC issue as Dan has pointed out?

Im not sure of much with it i was gonna put a new filter change the fluid and do a shift kit to see if that fixed but i wanna make sure its not electronic before i do i think it is so through advice on hear im sure i'll find it

Keep in mind they have the latest generation ATF.

General Motors DEXRONŽ-VI Global Service-Fill Specification

During early 2005 General Motors released a newly developed automatic transmission fluid (ATF) for the factory fill of all GM Powertrain stepped gear automatic transmissions. The new fluid provides significantly improved performance in terms of friction durability, viscosity stability, aeration and foam control and oxidation resistance. In addition, the fluid has the potential to enable improved fuel economy and extended drain intervals. Since the performance of the new fluid far exceeded that of the DEXRONŽ-III service-fill fluids available at the time it became necessary to upgrade the DEXRONŽ service-fill specification in order to ensure that similar fluids were available in the market for service situations. This latest upgrade to the service- fill specification is designated DEXRONŽ-VI.
Since General Motors introduced the first ATF service-fill specification in 1949 it has been periodically necessary to upgrade the specification. This upgrading process ensures that available service fill fluids are of an appropriate quality for use in transmissions that have been designed around the factory fill fluid performance. It should be noted that, as with previous upgrades, DEXRONŽ-VI fluids are designed to be backward compatible with earlier transmission hardware. More importantly, earlier type fluids are not forward compatible with transmission hardware that was designed to use DEXRONŽ-VI fluid, i.e. DEXRONŽ-III is not compatible with the most recently designed transmissions, and the use of these earlier type fluids could result in transmission damage. All current calibrations and certification tests are now conducted with DEXRONŽ-VI ATF. DEXRONŽ-III fluids should not be used for these applications where the owners manual recommends the use of DEXRONŽ-VI. GM does not license or support obsolete ATF specifications or the use of fluids that are being marketed against cancelled specifications.
All DEXRONŽ-III licenses expire at the end of 2006 and will not be renewed. Beyond that date GM will only support the use of DEXRONŽ-VI fluids for use in Hydra-Matic transmissions. Fluids sold in the market after that date bearing claims such as “suitable for use in DEXRONŽ-III applications” or similar wording should be avoided. DEXRONŽ-VI licensed fluids are fully backward compatible and can be used in all applications covered by earlier GM ATF specifications.

Thanks guns I will be sure to buy that when i do my filter and fluid change

I think that Dex VI is much more expensive than Dex III.

X2
This is a common problem that GM fixed in ’03 by using a shaft with hardened splines.
When my shaft went out, it would sometimes whine when trying to go into fourth.
Also when you lose 3/4 as opposed to TCC/no TCC there would be a more drastic change in rpm.

The trans shop confirmed my case by finding some fine metal shavings in the fluid.
They checked this somehow without removing the pan.
If I were to do it, I would put some magnets on the outside/bottom of pan, drive it for a while, then drop pan and look inside near the magnets for accumulation.

My first gut feeling was 4th hub shaft spline gears as well. However very valid points are being made.

Take it out for a drive with the selector in 3rd. After it is in 3rd and you are over 45-50ish, you should feel the TCC kick in and drop the rpm down by 300-500. After that happens, move the shifter to 4th and see the rpm'* drop further.

I'd expect the TCC to unlock, try the 4th shift and then relock. if your rpm don't go down more than 3rd with TCC lock, then most likely the 4th shaft is bad.

On the new Dex, it'* the current replacement for Dex III, however in our older transmissions there does not appear to be a great benefit from what I've heard. It'* about the equivalent of a synthetic fluid.

I will try that for sure boost,i was reading in another post i cant find it right now tho that said if your driving in 4th and you push the brake lightly the rpms will jump up.I did that and it does jump up about 500 rpms does that mean the splines are bad or is that the tcc out of adj