Is the size of the terrace sufficient? 4x4.5m

Kitchen belongs to the terrace, terrace belongs to the garden.
Do you want to carry everything through the house or just squeeze yourselves onto the terrace?
If there are small children or planned, should they play unsupervised in the garden?

4x4.5 are enough for a seating area with a grill, but for me the whole thing would already be too tight. The grill usually stands a bit further away, except in winter when everyone wants the warmth. Maybe on one side a bench that can be pushed directly against the railing. Just draw everything in. A sofa will no longer fit. You still need walkways.

What do you think, is the size enough?

You can't say that in general. It solely depends on what should be included and how many people are supposed to fit there. It's like with a house, some are satisfied with 100 sqm, others "need" 150 or 200.