Just off the top of my head (actually pretty barren up there)
I think if you add or subtract 1/2 of the lowest digit's value, i.e. 0.5, you can force the round to round up/down.

e.g. to force 123.xxxxxxx to round UP to 4 decimal places first add 0.00005 before doing the round.
to force 123.xxxxxxx to round DOWN to 4 decimal places first subtract 0.00005

For 3 decimal places add/subtract 0.0005.

No guarantees, but give it a try and see if it does what you want.
DaveBo out