Two important things:
1 - The rectangular ratio of a 10 x 15cm is 2 x 3 while the A4 format is a bit more square: 21 x 29.7. So if you don't crop a part of the image, you'll get a white border on 2 sides.
2 - Even if the 'collage' tool does not offer the option to choose the final size and resolution (pixels per centimeters/inches), it can produce the same result by enlarging the size of the final collage without losing quality. It's really faster to do than to explain how it works.
Start with creating a new empty A4 'canvas' (21cm x 29.7 cm)
Open the collage you have created and saved (with a pse file extension)
Be sure to see both your final collage and the empty canvas in the photo bin in the bottom in expert mode.
Select the empty canvas which displays in the editing space.
Click and drag the collage from the photo bin to the empty canvas on the editing space.
Use the move tool and drag the corners of the rectangle outwards to 'stretch' the collage and fit it within the empty canvas. You'll have to 'crop' if you want to fill the canvas, otherwise you'll get white margins if you want. Now you can flatten and save the result in your favotite file format (jpeg, psd...)
The reason why you can enlarge the 10 x 15 cm into an A4 canvas without losing quality is that the collage does use 'smart' layers' for the photos you have chosen to show. The collage shows a 'preview' of each photo but saves copies of all the original photos in full resolution and size. If you enlarge the canvas, all previews are calculated to use the original photos quality. If you have texts or shapes, which are 'vector' drawings, enlarging keeps the full details without any pixellisation or quality loss.