user14319: Let's assume I create and send a series of txs:
|------block A--------|------block B----------|----block C-----|----block D-----|
|Tx1......Tx2.........|...Tx3.................|................|......Tx4.......|

but then a fork happens and a block reorganization takes place:
|------block A--------|------block B----------|----block E-----|----block F-----|--block G--|
|Tx1......Tx2.........|...Tx3.................|......???.......|......???.......|.....???...|

Whatever happens to Tx4? Will it be included in one of blocks E, F, G or another block? Is it gone? Conflicted/invalidated? Do I need to resend it? If yes, can I spend the same inputs I did when creating Tx4?

user2306: 
Will it be included in one of blocks E, F, G or another block?

It's likely already included in one of those blocks. If not, it can be included in a block after that. 

Do I need to resend it?

Probably not. If block D or Tx4 was widely seen by the network, it should get into a block soon. (It might already be in blocks E, F, or G.)

If yes, can I spend the same inputs I did when creating Tx4?

Yes. When doing a reorg, the Bitcoin client first rolls the chainstate back to the first common ancestor.
The standard Bitcoin client actually includes a database for the sole purpose of rolling the blockchain back. (It's called an undofile.)

14319: <p>Let's assume I create and send a series of txs:</p>

<pre><code>|------block A--------|------block B----------|----block C-----|----block D-----|
|Tx1......Tx2.........|...Tx3.................|................|......Tx4.......|
</code></pre>

<p>but then a fork happens and a block reorganization takes place:</p>

<pre><code>|------block A--------|------block B----------|----block E-----|----block F-----|--block G--|
|Tx1......Tx2.........|...Tx3.................|......???.......|......???.......|.....???...|
</code></pre>

<p>Whatever happens to Tx4? Will it be included in one of blocks E, F, G or another block? Is it gone? Conflicted/invalidated? Do I need to resend it? If yes, can I spend the same inputs I did when creating Tx4?</p>
 2306: <blockquote>
  <p>Will it be included in one of blocks E, F, G or another block?</p>
</blockquote>

<p>It's likely already included in one of those blocks. If not, it can be included in a block after that. </p>

<blockquote>
  <p>Do I need to resend it?</p>
</blockquote>

<p>Probably not. If block D or Tx4 was widely seen by the network, it should get into a block soon. (It might already be in blocks E, F, or G.)</p>

<blockquote>
  <p>If yes, can I spend the same inputs I did when creating Tx4?</p>
</blockquote>

<p>Yes. When doing a reorg, the Bitcoin client first rolls the chainstate back to the first common ancestor.</p>

<p>The standard Bitcoin client actually includes a database for the sole purpose of rolling the blockchain back. (It's called an undofile.)</p>
