"Affects a large area around impact, soaking up to 20 bloons at once."
— BTD4 description
"Instead of glueing 1 bloon at a time, splatters 6 bloons at a time."
— BTD5 description
Glue Splatter is the final upgrade for the Glue Gunner in BTD4 and the second upgrade on path 2 in BTD5. It allows the glue to splatter in the impact, glueing 6 (20 in BTD4) bloons at a time. In BTD4 it costs $2550 on Easy, $3000 on Medium and $3240 on Hard, and in BTD5 it costs $1870 on Easy, $2200 on Medium and $2375 on Hard.
In BTD4 the Glue Splatter can splat many more Bloons than in BTD5. This is probably due to being a final upgrade and the fact that it has two glue guns instead of one. Since you can get the Glue Hose in BTD5, this makes sense.