2000 Honda Accord LX 4 cylinder
 

When I Start the car and put it in drive ill start driving and its fine. as i accelerate it does not switch into gear right away. Then when it does shift into gear , its a hard shift and the d4 light flashes. when i come to a stop then accelerate again its kind of bogging then once i get going it drive fine but there no real power but i can go on the freeway and it does fine. if i turn off the engine and start it again its fine. Now i think its a sensor and not the actual trans because if im on a hill and i start the car it has full power until it shifts and the d4 light goes on then it boggs only if i come to a stop.

How many miles are on the car? What's the history of transmission fluid? (ATF level on the transmission dipstick? ATF changes?)

Read the error codes & post them here. Since your D4 light flashes, you can go to many parts stores where they'll plug in a reader and tell you the error code(s). Get the actual code number(s); that's more important than what the store guy thinks you should fix.