XSL concatenation with nested if conditions

[sokol1273]

I have following xml

<string1>Y</string1>
<string2>aaabbbcccddd</string2>
<string3>I have to concatentate this</string3>
<string4>string4</string4>

I have to concatenate those four elements as follows. If flag Y exists for string1 then hard code 'Flag Y exists' + string2 (if exists) + string3 + string4 (if not empty) hardcode 'String4 not empty'. Please help me with the xsl phrase Thank you in advance