Tranmission Upshifts to 4th gear at wrong (low) speed

[PVibe2005]

Hello,I have 05 Vibe base model, has 45K, recently changed the spark plugs, and transmission fluid along with the engine oil (Mobil1 Extendend Performance 5w30).The problem I am experiencing is that, the transmission shifts up to 4th gear very quickly, so underpowered engine has a wierd noise, once I push the gas, the car becomes normal. If I lowered gas, it keeps the transmission at 4th gear, does not downshifts.What would be the reason of this issue? I appreciate for your feedback. thnx.

[RIT]

Check the type of T-fluid that you used, and also the level.