Think the 4100 is like the 4010 and very similar still to the 2025R. Hydraulic fluid check is to the left side and slightly under the seat. Think you can access the checker plug with the seat in place. Maybe have to raise it. Don''t remember. Don't recall that the 4010 had a check fluid window on it. Think it had a dip stick. The 2025R has no dip stick but has the check fluid circular window (which is virtually useless).
Think you have to remove the left rear wheel to remove the rod holding the lower 3ph arms, which goes right across in back of the part to remove to get at the hydraulic filter. You have to drain or to suction out the hydraulic fluid first, of course. This was the case on 4010. Don't have to on the 2025R. Hydraulic filter is actually as spin-on external one on the 2025R. It has a suction screen where the hydraulic filter was on the 4010 (and probably on the 4100).