The clutch fluid reservoir went dry I refilled and bled the air out now my bike doesn't want to downshift passed third gear once I get to fourth and need to downshift at stops.

reverse on most cars is a canary gear, (coal mine refr)
it is a square cut gear and has no sychromesh parts.
so loves to grind or even balk !, if clutch disk drags even the most tiny about
it means the bleed was not done fully and correct
dont feel bad, clerical clutch lines are super hard to bleed

you forcing fluid DOWN to the slave bleed (niPPLe-zerk)_
and air bubbles want top float up.
a war happen here in counter forces.
in the Chev , factory they used pressure bleeding to do it faster.

or you removed clutch linkage? and put it back wrong
ok told 2 of 4 causes.

not told the job you did only a CHANGE word
did you pull the transmission, says the blind guy me?

most cases the slave and master cylinder were bad
100% sure this ,, if the MC, reservoir was dry .
the fluid leaks out mostly while driving UNSEEN by you until
oops cap off reservoir and its dry or drops day by day.

classic manual transmission clutch failure but is 99% of the time
the clutch MC and slave leaked out all the Dot3 fluid
or went dry and not put back fresh and BLED , putting air bubbles there. this will result in CHAOS, random shift results.
no car told no miles on car told.
2nd gear loves to fail first in the city the insane ,1>2 shift
wears out the 2nd gear synchro ring a $15 part. (fixed many me)
are your sure Reverse never grinds or balks??????

if have see to that the cracked disc inside drags just a bit.
and this tiny drag (normal is zero) cause he most worn synchro fail
and grinds.
are you always topping off the clutch MC reservoir? with DOT3> brake fluid
make sure the correct gear lube is in the box. GL3/4 ? and correct viscosity, thicker is better for worn synchro rings.
if MC fluid is good
try first spec, gear lube. (do not guess learn what the car needs)
no car told.

Which transmission does yours have 4T65-E or 4T80-E ?

4T80-E
No Fourth Gear

Checks

Causes

1-2 Shift Solenoid Valve
?€¢
Inoperative
?€¢
Debris
?€¢
Pinched wires

3-4 Shift Valve Bore Plug

Misassembled

4th Band

Burned, slipping, missing

4th Servo Cover

Cracked

4th Servo Pin

Broken, seized

4th Servo Piston

Damaged

4th Servo Seals

Rolled, cut or damaged
4T65-E
Slipping or No Fourth Gear
Slipping or No Fourth Gear Checks
Cause

DEFINITION: A large engine RPM increase is noted in fourth gear, Drive range, with little or no vehicle forward speed increase. First, second, and third gear operation are normal. Shifts into fourth gear are soft or do not occur. Fourth gear ratio is 0.705:1.

Automatic Transmission Fluid Level
Low fluid level

Transmission Main Line Pressure
Low transmission main line pressure

Oil Filter Assembly
• The oil filter assembly (100) is missing, plugged, or damaged.
• The oil filter seal assembly (101) is missing or is damaged.

Control Valve Body Assembly
• The 3-4 shift valve (362) or the 4-3 manual downshift valve (360) is stuck or binding.
• Debris is found in the passages of the control valve body (301) or of the control valve body spacer plate and gasket assembly (396)

Case Cover Assembly
Distortion or damage occurs at the manual valve (404) or at the case cover (401), causing PRN oil leak.

Driven Sprocket Support Assembly
A plugged third clutch oil passage in the driven sprocket support assembly (609)

Fourth Clutch Assembly
• The clutch plates (500-502) are worn, damaged, or misassembled.
• A missing or not seated fourth clutch spring retaining ring (601)
• A damaged spring assembly (602) or piston (603)
• Worn or damaged fourth clutch piston seals (604, 605)

Third Clutch Sprag Assembly
• The sprag (720) was misassembled or backwards on the inner race (661).
• A damaged third clutch sprag assembly (653, 661, 719-721)

Clutch needs adjustment. if hydraulic drain the fluid from the reservoir and from the clutch slave. Refill and bleed as needed. If unsure get a manual, the procedure is very straight forward and not very difficult.

Open the bleed screw and pump all the fluid out of the system . Refill with metholated spirits and repeat the operation until it is also bled out . Fill with new fluid and bleed out till the reservoir is almost empty . Refill the reservoir.

the slave cylinder has a bad seal and all the oil is draining out
if you have not changed the master cylinder in the past 15 years or past 5 years replace both the slave and master cylinders and bleed the system

Check your clutch fluid level in the clutch master cylinder's reservoir. If low add brake fluid. If very low or dry, the clutch hydraulic system should be bled to remove air in the line. Bleeding is done down at the clutch slave cylinder on the transaxle.
I would not recommend using a lighter oil. It may cause early wear of the transmission.

This sounds like an issue with your clutch. It looks like you have a hydraulic clutch, so it may just need to be bled. As long as it is something in the clutch actuation, like the master or slave cylinders, it is not all that expensive. However, if it is a problem with the clutch itself, it can be an expensive fix. First thing to check is to see if the clutch fluid reservoir has fluid or not.

I believe your pressure plate is broken and at a minimum will have to be replaced along with the clutch disk.

Dot 3 oil.